Have you thought about the kindles? I think that would be the most economical route to go. What do you think your girls want to use them for? If it's just for reading, and internet, movies,and maybe a few games, the Kindle is the way to go. ( I think the only thing the kindle fire can't do is take pictures, but the kindle HD might)
We were going to go with the ipad, but the salesman at
Best Buy talked us out of it and we ended up getting a tablet that works off the Android system instead. It was a few hundred $ cheaper and it has more memory. So far i have been happy with it. But I certainly wouldn't want to buy two of them because it was still pricey.
Hope that helps.