I don’t have kids so I can’t make a valid kid recommendation, but the App Store is filled to the brim with crap apps I wouldn’t want anyone to put on their phone. Not necessarily directly harmful apps but there’s just tons and tons of garbage in the App Store.
It may be “free” but that doesn’t make it ok.
The App Store primarily is filled with junk food. Brain rotting stuff. There’s some useful nuggets hidden in there, but for the most part it’s the world’s largest SuperFund site for the mind.
I wouldn’t let them run free in there.