Amazons Fire tablet allows you to block apps like Youtube with a password (or completely hide them), but I havent ever tried to "hack" them , so dont know how effective they are
Regarding YouTube, the YouTube Kids app lets me block videos and whole channels, and it's great. But there is no equivalent functionality in the main YouTube app. On various devices (e.g., Roku, Google Nest) the kid sometimes manages to find some YouTube channels I'd rather they didn't watch. But I have to manually intervene each time, I can't just block those channels from being watched or recommended in the future. YouTube/Google obviously know that it's valuable to block videos/channels, since that feature exists in the kids app, yet they omit the feature from the main app, even for a paying customer like me. It is obnoxious.