here's a few other sites that talk about court sizes for kids https://granitestatekids.com/10-and-under-kids-tennis/
http://www.charlotte.usta.com/USTA_Jr__Team_Tennis/Quick_Start_Tennis
https://www.classicturf.org/what-is-a-regulation-size-tennis-court/
In iD you can enable measurement panel and see measure the dimensions of those courts.

You could add max_age=8 tag to these courts if you feel that's necessary.. 🤷‍♂️