There are two options that Brock could take here.

He could be an adult and give her the candy, then take this stupid flower-shaped pillow back to his fort.

Or he could lob the pillow right back at her like a baby.

Brock takes the third option, which is to say he makes a face at her, then quickly goes back to the fort with the pillow and the candy. This is still acting like a baby if you were keeping track.