If it were up to most kids, they'd watch YouTube videos all day long. Even if they're watching strictly kid-friendly videos on YouTube, kids still have access to plenty of content you probably don't want them watching (like endless unboxing videos and ads).
Jellies is an app that only offers handpicked videos that are safe for preschoolers through elementary-aged kids. (Seriously, each video is watched and chosen by an actual human grown up.)
There are over 5,200 videos to choose from, but you can set your child's age range to ensure they only have access to videos deemed appropriate. Turn on Parent Mode to control what your kids watch. You can also set a timer so there is (hopefully) less of an argument with time's up. And you don't have to worry about ads because there aren't any.
Try Jellies free for 30 days, and then it's $4.99 per month.