Now I am not obsessed with the 'I am your father' spoiler, as that is not the reason I say watch the OT first, cause essentially I think whatever trilogy you watch first, the other one is pretty much spoiled, so either way, the drama from one trilogy is drowned out by watching the other one first.
The reason I say the OT first is simple: ANH should be seen as a standalone movie. If the kid loves it like we did, then you show him ESB & ROTJ, and tell him this is more of the story of the adventures of Han, Luke, and Leia. If he loves those movies, then you say there is a backstory of how Darth Vader came to be, how ObiWan was a young jedi, how The Emperor came to power. You have to fall in love with one movie to fall in love with a movie series, and I am banking ANH still appeals to the masses in that respect.