2 daugheters received an iPad. Both are on my iTunes account. How to differentiate the two devices in the Game Center?

So my two daughters (aged 7 & 9) each received an iPad for Christmas. They are interested in the same games and I installed them on their device. Because I registered both of their iPads on my iTunes account, when play these games they are "me". I would like to differentiate the two so they are each playing their own account. Do I need to create seperate iTunes accounts for them? If I do that, I can't install the apps that I have already bought. There must be a way! Any help would be appreciated.