Last year for Christmas we got my 4 year old a Kiddiezoom digital camera. Technically it wasn't at the top of the tree by any means, but it took quite good 2MP pictures, and was very robust. She's taken quite a few decent photos, plus some that are 'interesting' because of the way she see's the world... plus a bunch that are just 'strange', So i'd recommend one of those as a gift.. It also taught her that she needs to look after a gadget etc...
And she's grown (in age, responsibility and ability) to the stage where my wife lets her play with her samsung android without batting an eyelid, and i'll let her play with mine without getting too nervous (as long as i'm there).
The tablet you mentioned looks like the sort of thing i'm looking at, althought its probably not going to happen this year as she has lots of other stuff on her list.. but she does have a birthday next year, and its not long before mine so it may end up as a 'shared gift'
The Nabi looks like a nice bit of kit although is a bit more than i want to spend... but if it comes down in price it may be a real option.
Also consider the Blackberry Playbook.. I know its not android, but a lot of the apps are available on it, and its very nicely built and on sale through curries and pc-world at the moment. My Father has one and i'm really impressed with it.
Anyway, if you do go for it, then let us know how you get on with it, as we'd really like to know...