Good morning, I would like to help with your question today.
One great application for children your twins' age is Disney's "It's a Small World" app, available on Apple's website: http://itunes.apple.com/us/app/its-a-small-world/id484670255?mt=8
It's for both iPad and iPhone. I don't have this one for my twins yet, but I am pretty impressed by what I've seen. In fact, I'm glad I got your question, because I think I'll download this one today:http://www.youtube.com/watch?feature=player_embedded&v=hUSIe3zh1C8
Here's another good app, it's from the popular book "5 Little Monkeys": http://itunes.apple.com/us/app/five-little-monkeys-collection/id519896820?mt=8
You can never go wrong with the alphabet; and this one is only $.99: http://itunes.apple.com/us/app/letters-to-z-for-ipad-from/id364047351?mt=8
Now this one is my absolute favorite. I have it, and my 18 month old twins both love it (But they love anything Elmo or Sesame Street for that matter); "Elmo's Monster Maker": http://itunes.apple.com/us/app/elmos-monster-maker/id350361202?mt=8
Here's a taste of the fun packed in this one: http://www.youtube.com/watch?feature=player_embedded&v=9jMfaEvkNTg