Nommons was just the ticket. Nommons isn't necessarily a math app or a game app; it's both, and you can position it to your kids the way they want! It's a game that plays like a side scroller in space, with cartoony graphics, aliens to avoid, candy to collect, etc. but you also earn points, which can be used to enhance your spaceship, by answering math questions.
You can select addition, subtraction, multiplication, and division at startup, so the app can grow with you and your child as they learn more, starting in kindergarten and into elementary school. Nommons: Math Universe is marketed as suitable for ages 6 to 8 and that's about right.
While there are power ups to be bought, you only use virtual currency, earned by progressing through the app. I like that there are no in-app purchases. I've been burned before, not by my own kids, but I once lent my iPad to a friend's son, and he racked up $75 in charges in a matter of minutes! I swear I didn't give him my password, but I must have logged in to the App Store within Apple's predetermined grace period, so no password was required. Fortunately, Apple refunded the money and I've totally disabled In-App purchases, which can be a nuisance, but better safe than sorry. So, it's nice to see a children's app without such dangers.
Speaking of iPads, Nommons is a full-fledged universal iPad and iPhone app, so it takes advantage of the entire screen. Given the $2.99 price, it might be worthwhile having a free version so people can try before they buy. As the name suggests, the publishers, YummyYummyTummy, could easily add more universes, like Reading, History etc., and have a full line of educational games!