I think the key problem is that attention is the only currency that many children have access to. Many many many kids have mobile devices now, along with tons

I think the key problem is that attention is the only currency that many children have access to. Many many many kids have mobile devices now, along with tons of free time and a strong desire to play games. What they don't often have is access to a credit card to purchase games or do in-app purchases. I think that a big part of why so many games lean on ads is because it's essentially the main currency that kids have access to: their own time.