Sandlot slang for 'mouth'. c.f. pie hole. Appropriate for use in the company of young children. 'Snack Trap' is a semi-generic name for a plastic container often seen full of Cheerios in the hands of 2-year olds.
Father: I really don't want to read this 'Thomas the Tank' dreck anymore.
Mother: Shut your snack trap, daddy, I've read it three times already today.
Child: We went to the Exploratorium and daddy got me some Jelly Beans!
Mother: Jelly Beans?! Really?
Father: Watch your snack trap, kid, that was supposed to be a secret!