that's what the YouTube Kids app is for. you log in with your Google account and you create the kids accounts. you can select content by age and blacklist channels or you can whitelist only. Whitelisted only is probably the only way you can block some of the more annoying channels like Vlad and Nicki, they seem to have literally dozens of channels all full of the same drivel.

additionally, iPads have strong parental controls. you can limit time overall on the iPad and you can limit time on specific apps.