I synch several iPads and other iDevices to one account without problem. Each device has its own profile on iTunes, so I synch only the apps and content that I choose. I even synch my husband's iPad for him, because he hates iTunes and avoids it at all costs, and he's in IT and usually does all our tech stuff, lol.

By the way, OP, you don't have to sort through apps much once you've set to manual synching on iTunes. If you just leave your granddaughter's profile toggled on to updating specific apps, you only need to toggle on again to any new app you want synched to her iPad. iTunes remembers thereafter.