Possibly as "wee-wee" for his own "tummy banana" and "boobies" for Petunia's "tits." Frankly, I doubt that at ten Dudley would have been through anything resembling sex-ed at school, if only because I can't see his parents signing the permision form.

BTW, here is a website that will undoubtledly help you with some Brit-speak (http://www.peevish.co.uk/slang/).