Why couldn't Sam and Tucker see Youngblood in The Fenton Menace?

The scene is included for better context. I've had this question for a while. As stated in the episode, only kids can see Youngblood (hence the name "Youngblood" and the reason Jazz couldn't see him until she acted childish again). Why do Sam and Tucker insist they can't see him in this scene? They don't think they're adults. Unless, of course, they were lying, which would make sense since Jazz and Danny's parents were there, but at least then they'd know he wasn't crazy. Or maybe he really WAS seeing things at that point? I'm probably missing something obvious. Who knows? What do you think?