Depends on how much you want to spend as well,
Here are two good set-ups that really are not that expensive if you plan on taking them out regularly.
1)Hyperlite Motive 119 with mini squirts, this is the first set up that I put my boy on and he loved it. This board is for the little guys 5 to about 8 or 9.
2) The next board is the Liquid Force Nemesis 124 with the Micro Series bindings. This board will give them a little more room to grow because it is a little bigger. I would recommend the second of the two, my boy just turned 7 and is now on this board. He will probably be on this one for at least 3 more years if not longer for progression.
Good luck and have fun with them, they will dig it.