Holden thinks this is funny because
That's the same place I used to like to skate when I was a kid (chapter 16).
Holden is generally so alienated from other people that he finds it strange that anyone should like doing the same things he does, or once did.
However, it is true that Holden does get on with children, he finds it easy to talk to them, and he is especially close to Phoebe, his kid sister. Moreover, he is always reminiscing about his other younger sibling Allie, who died, and the one time he feels any joy in the novel is at the very end, when he's watching Phoebe and the other kids on the carousel. In fact, he is quite overcome by his feelings:
I was damn near bawling, I felt so damn happy, if you want to know the truth. (chapter 26)
It is only with children that Holden ever feels content and secure. This can be taken as a sign of his own immaturity, and also, quite simply, his yearning for a pure, carefree, innocent world - like the world of children.