There are Story Chimes read-along story apps for iPad. Stories include: Noah's Ark, The Very First Christmas (the Birth of Jesus), The Story of Easter, Passover, Exodus and Hanukkah. I'm not sure what else is out there. I know there are apps for GLO Bible (which provides multi-media tours of certain biblical locations, interactive maps, video, and pictures), a number of Bible trivia game apps, a live video feed of the Western Wall at the Temple Mount in Jerusalem, and apps that help you light a Menorah for Hanukkah and play the shofar and David's harp.
Besides that, the best thing to do is search the app store on iTunes and see what's out there. If you find enough apps for sale to cover the majority of stories that are in your church's scope and sequence, then go for it. Otherwise I'd say that ther purchase of all of the iPads might not be worth it. The tough thing is finding enough Bible related apps for your pre-K kids. Hope this helps.