First in Destined rivals saga the first time you play as Goku go to Kamis lookouut and do King Kais challenge. you will go up against Grandpa Gohan and you must win. You will get him and another Z item... save that for later. The second item to unlock kid goku is at Majjin Buu saga: Two Majjin Buus go to mountain road and do yamchas challenge. You have to win this battle or all you get is money. When you win you will get another Z item, go to the item shop and buy Z item fusion. Next go to Evolution Z and item fusion. fuse the two items you got together and you will get kid goku.