to give an idea,
the TT school from Seiya Kishikawa (ex-JNT member and WTTC doubles medalist) in Ariake [Tokyo]
https://seiyatts-ariake.com/course/ has classes for juniors
the price is the second line in blue, and for 20 group lessons a week ( 5 times a week) it's 16,000 yens per month
I think the price is in line with other schools, but here is the catch:
the school is rather new ( 1 year old) and is not easily accessible (its much easier by car than by common transportation), thus at the moment they are very short of students. I know even a friend (adult) who struck a monthly group deal, but in practice he is the only one going at his time slot, so he's paying group fee for personal lessons with top coaches, sometimes Hidetoshi Oya himself (google him). Actually i do hire Oya as well, but pay the full fee (like 3x or 4x more) as I don't have time to go to Ariake, and I ask him to kindly go near where I live
I don't know how long the situation will perdure, nor how it is for junior classes, but it is probably not so crowded so your kid can receive relatively more attention.
The other very nice things about this TT school is that it is located in a big sports complex, you can also have access to showers (lacking in most TT schools !), and probably also access to gym, swimming pool, maybe an extra fee is needed, you would have to ask them.
There are many other TT schools in Tokyo, for example Yoyo Takkyu
https://www.yoyo-takkyu.com/
who has among its staff some recently retired ex-professional female players like Yuka Ishigaki (chopper), Hiroko Fujii (chopper ?), Misako Wakamiya (attack).