first of all I don't advocate giving really young children cellphones or tablets BUT incase you have to then.....
here are some cheap phones that they can play around
a friend in china said these where made for the elderly for basic day to day use
https://www.mi.com/hk/product/poco-c75/?skupanel=1
https://www.mi.com/hk/product/redmi-a3/?skupanel=1