My take -- for what it's worth:
There's three factors in play here with to the OP's dilemma -- they break down to in-game factors, after-game factors, and peace of mind factors
In-game, in a tournament, you are there to win. Your opponent knows this, and is there for the same reason. Playing to win your match is the whole aim for both of you, and so long as your play is legal, then anything goes. Yes, psychological factors matter, and taking advantage of your opponents mental weaknesses, as well as their physical limitations, can all be part of the game. So if it's legal, and fair, then you should go for it... such is the logic of competition.
After-game however, you stop being a competitor in the midst of a competitive battle, and start being a human being. And being a decent human being is far more important than being a fierce competitor.
I have a couple of rules I follow in regards to TT. The first rule is "What happens at the table, stays at the table" -- this rule applies to my opponent's conduct though, not my own. I have this rule to remind myself that's it's all just a game, and I shouldn't carry any grudges about other people's tactics I may have elsewhere into my own life. Leave any resentment you may have about a match at the table. Do this literally. Literally imagine yourself walking away from it in your mind as you're waking back to your chair and kit bag. it really helps me feel better about the times I lose in unpleasant circumstances I didn't like.
The second TT rule I have is just for myself, which is "Never do anything at the table in the name of winning that I might be ashamed of afterwards". TT is a game for most of us, and a profession for a few of us. That means most of us look to it for fun, and the rest look to it for income, including myself -- neither of those motivations however excuse us being manipulative arseholes, or treating another human being with contempt. Yet in the heat of a match, sometimes it can be really easy to forget that. It's really easy for competition to bring out the arsehole in all of us. Winning is a great feeling for me personally, but winning at the cost of someone else's peace of mind, happiness or self-esteem is not a winning feeling whatsoever. If I haven't played honorably, then I just don't feel great about it afterwards; those bad feelings also last a hell of a lot longer for me than the buzz of winning does -- I learnt that one as a very young man, and it's something I carry with me always. To paraphrase Ender's Game: Yes winning matters, but HOW we win also has to matter.
This brings me to the third factor, which is peace of mind.
You won your match against the 5-year old, you used legal, ethical and legitimate tactics to do it... And you utterly kicked their arse. You killed it -- so well done, enjoy your win.
...but you CAN'T really enjoy the win can you? You're here asking us about it, so I'm guessing you're looking to justify your actions in the aftermath, or else lay any lingering feelings of discomfort to rest.
My advice is, don't lay them to rest. Not yet anyway.
If you're still bothered by the way you played, or about the outcome, then congratulations -- this means you have a conscience, and are therefore a decent human being. That simple fact is worth a thousand times more celebration frankly than you thrashing a 5-year old in public. (It's tragic how rare having conscience is becoming in this world, but oh well....)
To reconcile any feelings of discomfort you may still have, I recommend you commit yourself in future, to doing what I do in such situations (something I actually encounter every single week at my local club). You should aim to win, but aim to do so honourably. You do that by pulling your game in a little bit, and by giving the child a sporting chance.
If the child is genuinely better than you at aged five, then they will whip your arse anyway. Take the loss and resolve to get better yourself.
If you are the better player, and the child has an obvious weakness such as the short serve, then try doing what I do, and use the short serve to win points when necessary, and to ensure you win the match... but also mix your game up and do other serves too. Not just to give the kid a chance to win a few points, but for other reasons too.
Throw in some long fast serves as well. It can expose other potential weaknesses in their game, which is good practice for tighter games for you, and good feedback for them, as their coach will take note of it, and have them work on it. More likely though a low fast serve will be smashed back at you, as driving fast balls is what those young kids typically practice the most. In which case, the kid won a point -- that helps them feel better, and gives you a window into their own skillset -- which is also good practice for more competitive games. Straight away though, you now know how to win a point, and how to lose them... So control of the match is now entirely in your hands, and you have the option to artificially make the game closer than it would otherwise be.
This is what I do with the very young players I encounter in comp matches-- I either lose fair and square, or I JUST win the match by pulling my punches, so the margin is closer than it should have otherwise been. I'm also not afraid to offer the young kids a little advice during a game, or to praise them when they play a good shot, or make a good tactical decision.
I do all the above to live with myself afterwards, but also to ensure that my young, underage, emotionally vulnerable and obviously inexperienced opponent doesn't feel crushed at the end of the game, but (hopefully) instead feels encouraged. I tend to find the very young kids feel okay with *just* losing to an experienced adult -- especially if it's in a "tight" five-setter.
Every time I do this, typically the kid walks away encouraged to try harder next time, and commit themselves more to practice, so that they can kick my arse next time... And I meanwhile, get to advance in the competition to the next round, and walk away from the table knowing I won without destroying somebody's self confidence at the same time. And that makes ME feel good.
Sure I'm playing with fire by doing this. Occasionally the kid improves during the match and ends up winning because I threw a game or two. But I can live with that, because I'm still being beaten by my opponent on the night, and I'm still behaving in a manner consistent with my own ethics and guiding principles.
The *real* proof of the benefits of this approach however is that every time I win this way, the kid's parents and coaches typically thank me warmly for it after the game. They don't want to see their kid getting slaughtered, anymore than I want to do the butchering. They appreciate I treated their kid gently, and let them down gently, without destroying their motivation, or robbing them even a little, of their love of the game.
There are people out there who believe that winning is all that matters, and -- judging from some of the comments in this thread -- such people genuinely don't give a steaming shit about other people, or about a child's feelings, or about anything else than themselves once they have a blade in their hands. They are apparently of the mindset that "once the blade is in, twist it". I encourage you NOT to join them.
I have no idea whatsoever what internal psychological forces help somebody justify to themselves destroying a child's self confidence or feelings of self-worth, or else rationalise or otherwise ignore the damage their actions can do to others. Maybe that's all they know and understand...
I however don't want to normalise or rationalise connotational or situational contempt towards children, and I certainly don't wish to remain silent in the face of anybody who does. I personally don't want to behave that way towards anybody EVER frankly.... not even in jest.
Either everyone deserves respect *all the time*, or nobody does. I recommend you be wary of anybody who argues otherwise. They're trying to justify the idea of treating people with selective contempt, just so long as it never happens to them.
I sell blades for a living, so yes, in a way my approach is also self serving, as it helps ingratiate me with potential customers to some small degree... But I can live with that. TT isn't a huge sport here in Australia, so anything I can do to help keep kids playing the game is healthy for the sport in my book. We need more gifted and motivated 5-year olds playing this sport we all love so much, not less. Treat them with respect and kid gloves, and they'll stick around. Thrash them every time you meet them however, and they probably won't.
Hope this all helps you. Good luck with your future games, thank you for having the guts to ask an ethical question on a public forum, and God bless you for having a conscience, and being willing to guide it with outside advice, and protect it from outside advice. All the best for your playing career! 🙂🙂