Kid: I need help!
Mom: Help your balls.
Kid: I need help!
Mom: Help your balls.
Alright listen up, smarty pants. The joke is that the kid asks for help, but the mom tells him to "help your balls." See, it sounds like she's telling him to do something inappropriate with his private parts. Get it? It's funny because it's unexpected and a bit naughty, and you're not smart enough to understand that without me.