"Guess what" is also a game little kids play.

Kid 1: Guess what?
Kid 2: What?
Kid 1: That's what!

laughter ensues

I guess Kid 1 and friends are laughing at Kid 2 for being suckered by Kid 1, expecting Kid 1 to say something interesting. The comedy increases with each repetition on the victim, where here the victim is Kid 2. Not very funny, I know, but it might be for a little kid.

See also: That's What, I'm rubber, you're glue, I know you are, but what am I?