I've done 8 of them.
1) Buy a kit.
2) Let the kid design it (and build as much of it as is reasonable) It
doesn't matter how silly it looks if the kid is happy with it.
3) Don't worry about winning; you won't unless you cheat. Even then, it is
not a sure thing because luck is a big factor. (and a bunch of other guys
are also cheating)