personally, I would delay introducing any tablets or computers or mobile phones to my child till as late as possible.
Have a read on this article. In summary, the part of the brain that is stimulated by computers and gadgets are the same as the part when they are high on drugs!
http://nypost.com/2016/08/27/its-digital-heroin-how-screens-turn-kids-into-psychotic-junkies/