I think it's a good idea.
We bought our daughter a cheap Android tab when she was 2 and a half and she was able to learn how to use it very quick. Now she is nearly 4 she is very used to the Android system and can quite happily watch netflix/play learning and fun games.
There are more and more valuable learning tools for them and it can be quite a benefit for them.
As long as the content can be controlled and no unauthorised app purchases can be made go for it.