I've got a 4 y/o girl and a 6 y/o boy and the iPad has been a GODSEND when it comes to keeping them entertained on road trips. There are so many kid's apps that it can be hard to sort through them all, but here are some I've found work well with my own kids as far as a) keeping them entertained for a decent amount of time b) being at least somewhat educational and c) they can use on their own (i.e. from the backseat...)
1. Looking Glass - short (around 3 or 4 min) videos for young kids. No words, just music. Subscription that gives you new videos every week (although my son wants to watch the tow truck video on repeat...) Great because they can keep themselves entertained without my having to jump in.
2. Scribble Lite - coloring app. Super simple but keeps both kids entertained for relatively long periods of time.
3. Smack Talk - OK, this one doesn't have much for educational value, to be perfectly honest it can get pretty annoying, but it makes it on the list due to the sheer time my kids are entertained by it. This app lets kids record their own voices then manipulate the sound. They can use this thing forever without getting bored.
I'd love to know what other apps you've found work well for all those things with your own kids around the same age? I'm always looking for new suggestions since we travel fairly often and keeping the kids occupied has a LOT to do with the overall success of the trip