Part of the reason why YouTube is so popular is because it has essentially become a native app on many devices due to manufacturer pre-installation. To use your screaming child in Wal-Mart justification from earlier, parents who encounter this "issue" for the first time with their first child aren't going to go searching for an app in the marketplace to quell the shrieks. They are going to do that fastest most natural thing to them - type in kids songs or kids videos on their natively installed YouTube app on their phone. And that's where the algorithm begins. Then they say "hey there's even more kids stuff on there - I'll just use that". That's why YouTube Kids isn't going to succeed unless major adjustments are made. It is floating around on its own desert island. The point that was made earlier about children's progression into teen years and wanting to flee that platform at a certain point of maturity was very valid too, and from YouTube's perspective, they probably want to keep kids in the app that can eventually behaviorally advertise to them. This is all about user experience and natural flow. Any justification for a pay gate, login gate, or age gate on YouTube isn't going to fly, as a percentage of people will bounce. YouTube doesn't want to turn end-users away from the platform for any reason - just creators
If YouTube does this, another platform will capitalize on that decision. They know this.