I'm on the fence about letting kids use apps with ads. On the one hand, my kids don't notice them at all. Don't notice, don't click. On the other, there have been problems reported with some developers using ad services that didn't properly screen the ads for inappropriate material.
So, I think it depends on the individual app and developer.
Balloony Word is a good example. The version with ads is free. The non-ad version is $1.99. For $1.99 the app is decent. As a free app, it's a nice download...and the only ad I've seen on it is for booking.com. This review, then, is for the free version.