A: Want some gum?
B: Ooh, yes, please.
A: CHEW MY BUM!!!!

Presumably the victim is too shocked by the voracity of the comeback to take you up on the offer.

A variation of the offer of chewing gum, deftly switched with an invitation to chew your bum is available to those who have sweets.
A : Want a sweet?
B : Of course I do! I'm a kid. That's what kids do! We want sweets!
A : Suck my feet!
B : Feet! That kinda came from nowhere. I thought you were going to give me sweets.
Here are the only five other sweet / body rhymes...
Want a Mars? Kiss my arse!
Want a Snickers? Piss your knickers!
Kola Kubes? Snag my pubes!
Reece's Pieces? Suck my faeces!
Butterkist? Anal fist!
Can you think of more? If so, write them down and post them to your mother, see if SHE thinks your potty mouth is funny.