YEP!
I'd LIE and tell them it's not for them! Unless, of course you've already let them know it is after the little one came running in the house. Then suprise them w/ the truth on Christmas. In fact, maybe even remove it from the garage and ask a neighbor to hold it for you until that day so it's more believeable! -?- lol, have fun w/ it, don't stress yourself!
It's best to wait, I think-whether they know it's really for them or not.
The next gift(s) you should be getting are the helmets; goggles and pads!! (I own a 4 wheeler myself and it's no fun when you fall if you're not properly equipped!)
The best place to get your helmet and goggles would certainly be on-line, I bought mine from E-Bay!!