You can use Newpipe (it's open source), and there you may simply subscribe to the channels you want the kid to watch and they'll only watch those channels. Ther

You can use Newpipe (it's open source), and there you may simply subscribe to the channels you want the kid to watch and they'll only watch those channels. There's nothing as home screen there but the kid can still search for a video using a query so it might just be the solution you are looking for.