"Kiddie" type products/tablets will be outgrown... QUICK. So I do not recommend those things.
Look for one that will grow with her.
We have a family, iPad. The 1st generation version. It has lasted a long time and is well used and even my son, who is now 6 years old, can use it proficiently. He knows how to use it better than me, and I am an adult. And of course we have "rules" about it. Which he is good about.
Even in school, the 1st grade level has iPads to learn from. And the kids have computer class in school.
Personally, I like the iPads or the Nook HD. The Nook HD is expandable and can handle flash websites, for example. Any Barnes and Noble, has a Nook counter and FREE customer service for it.
For your son to have a tablet, you as the parent ALSO has to know how it works and what can/cannot be done on it and how to download things to it and have a password on it.
AND, being it is a pricey apparatus for a young child, you gotta make sure he will know how to take care of it.