I'd say a something close to a clown outfit is your best bet. Don't completely sell out by going all the way and doing the rubber nose and big shoes, though. You can get big stripped overalls at costume stores. Put those on over a plain colored t-shirt. Then gel your hair in all kinds of crazy directions. I've seen it, it works
Absolute rubbish!
Clowns belong in a circus and you're being hired as a magician - not a clown. There is a dependancy on the age of the children too. Clowns can be scary to the very young and are 'old hat' to the older children.
Dress smartly and then rely on your skill to get you through the show. Most of the best childrens entertainers (and I did say most), wear nothing more than a plain shirt and trousers with a coloured jacket with some fancy gold stars stuck on.
Avoid costumes.