Likely this will depend on which specific platforms you are currently using. Your primary router likely already has some of this built in, where you can block specific domains, and set time-limits based on specific users. For youtube, I might recommend enabling youtube-kids on their devices, however that isn't really foolproof, and can be really limiting in the kinds of videos they can access. I've found that it blocks some really educational resources for my kids. Amazon tablets have a similar feature-set (but are similarly limiting). Microsoft Family has some more advanced parental control features. Your "internet security" provider might also have some solutions, for this I use Norton.
Best advice I have, depending on the age of your children, is to talk with them about safe internet usage, and trust them to use it in their own best interests.