I need help with Spanish, imperfect vs. preterite!?

It depends on what is what you intend to say, to convey. If what you want to say is that when you were a boy you used to laugh about life or things, etc., then you should say: "Cuado era nino, yo reai'a". But if what you intend to say is than while being a boy you laughed at something that happened once, then you say "cuando era nino, me rei' or "de nino me rei'".