The unnamed child only appears in the episode "Scaredy Pants" and is the child of the unnamed parents.
He is a light green fish. He wears a brown jacket with a belt and a gray hat with a red stripe and carries a treasure bag. Since he only appears in a Halloween episode, he most likely is dressed up like this as a cowboy costume.
He only has the line "Trick-or-treat!" and when he says it, he scares away SpongeBob SquarePants. His parents then explain to him that it's alright because SpongeBob is just scared of him.