The iPad gives the most immediate as well as long-term benefits as it:

• is far more intuitive so can concentrate on the task at hand: Androids and Playbooks still feel like computers. Nice computers that are relatively friendly, but still computers.
• has far more apps available, especially in education and for kids, with far better quality/safety control
• has very well-integrated and fine-grained parental controls that can protect against a lot of different potential hazards
• is tough in itself but has thousands of cases available, many of which are made to be fairly child resistant (and one or two of which can protect against explosions, dropping from 10th floor onto concrete, dropping in water and so on, though that is maybe not the same as proof against 7 year olds)

I have both iPad and Android (eee) tablets. Much as I try to love the Android as it accords better with my desire for flexibility and diversity, it is not even close to the iPad in terms of friendliness and usability. Superficially similar but in another much lower league, despite superior hardware specs.

Might be worth waiting for the new OLPC tablet, if it ever becomes possible to get your hands on one: incredibly cheap, innovative and child-proof, though still an Android device with very low specs. See http://www.wired.com/gadgetlab/2012/01/olpc-tablet-finally-arrives/

Agree with Jon about the iPad, mostly on account of the apps. We get a lot of mileage out of even the free kids' apps. And sometimes they find new ways to use apps for grownups too.

I will agree with Eric - those devices typically are a distraction for kids. single-purpose tools. Our experience is that given the device that can do "many things" distracts child at that age and doesn't let them concentrate on one thing since [s]he knows - there are toys and other things available on that same device.