If your child put a dollar in a soda vending machine at school and the vending machine then took their picture, that would be a similar situation. Arguing that it's not the school's fault your child had their picture taken is silly: the school put the vending machine there. The child didn't know the repercussions and can't make an informed choice.

So if YouTube puts a big ad in an app exclusively for kids then how is it not their problem if clicking that ad then tracks the child? Who cares what website does the tracking if YouTube deliberately sends the child from an app meant for children to a website that tracks them.