Where I've seen a kiddie pool at a hotel, it's usually for toddlers. Bigger kids are in the "main" pool. It usually takes a pretty big resort for there to be an "adult" pool (sometimes with swim-up bar, which is awesome).

I've stayed in a few hotels that have an hour early in the morning and/or one late at night when no kids are allowed in the main pool.

At a hotel, I kind of expect the kids to be everywhere, but I'd also add that Marco Polo is a dumb game and they shouldn't play it in a crowded pool to begin with.