My boy is three years old. I picked up the iPad mini for him and the 7" form factor works very well. He is also trilingual so using this to help with his English has been very useful.
The unit is 16 GB which has been plenty for us. Every couple months we change the movies to the flavor of the month which keeps him amused on long flights (12 - 14 hours ).
Value for money most reviews list the Nexus 7 as the tablet of choice. What ever tablet you choose, a good quality cover and screen protector is essential. Even when my boy is trying to be careful the tablet gets it freefalling time.