Whats ironic about Piggy's statement, " 'like kids!' he said scronfully. ' Acting like a crowd of kids!'"

This statement is ironic because they ARE a crowd of kids.

Piggy is angry because they are acting like kids, but they are kids. Piggy expects them to act more maturely than they are capable of doing.

It is statements like this that contribute to Piggy's being an outsider. He continually places himself "above" the others and criticizes their behavior, much like an adult would.