My 5 and 7 year old really enjoy Lightbot (there's a limited, free version available to try) for basic logic/programming and DreamBox for math. Dreambox is probably slightly advanced for a 4.5 year old, but not much and they claim to modulate the difficulty based on the student's actual performance.
They also watch PBS Kids and BrainPop videos on the device, which is basically watching TV, so we limit that a fair bit. We don't limit the educational games quite as much.
They also got into scratchjr (from hour of code, sort of) recently. It's unclear how much "legs" that will have as it seems fairly simplistic to me, but it also is fairly open-ended and allows them to do some creative expression and storytelling with programming as a backbone. The programming part is frustrating to them at times, because it's so fiddly. I'm a (n ex-) computer programmer and it would drive me nuts, but the kids stick with it and get little stories/skits made on the iPad that they're then extremely proud to share with us.