DragonBox helps children learn algebra using a very novel technique. It comes in 2 versions: a 5+ and a 12+ version.
I've only bought the 5+ version, not the 12+ version, so can't comment on the latter. But the 5+ version definitely helps you learn basic algebra concepts by pattern-matching, and I've found it to be a very cool concept. (You may not get far enough within the 15-minute trial window to get the concept, though, so be prepared to actually have to pay first.)
It's been a while since I've played the game (I stopped playing when I finished the game all the way through), so I don't know if the current version has ads or in-app purchases. But certainly, when I played it, it had neither of those antifeatures.