In a child's mind, they haven't yet fully decided how things behave and react, so they are willing to try things to see if they've changed (because in fact often they do in their own mental models, as well as the resolution of those models.

If you think of it as a regression tree, their models start out fairly shallow and then slowly get deeper as they age. That's a square thing. That's a book. That's a book about animals. That's a book about marine animals. That's a book about marine animals that I like. That's a book about whales.

In trying to solve the multi armed bandit, they're willing to explore to try to find better rewards because in their own world there are often better rewards.

If the game shifted to having a different alien be the best one half way through the game, I'd be curious how quickly the more maturely cognitive kids did. It's possible that becoming fixated on something (the way adults are) can ultimately limit your abilities in a game. In adulthood, "creative" solutions can sometimes be better than more "standard" solutions, but incur more risk.