Odd, it looks like there should be enough room. The poly on those is usually very thin, from what I've seen. You may be better off carving a bit off the back of the ear, rather than trying to take some from the head. The idea is to get the earring on display, right?
I would suggest something in the 100 series of bits on a lower speed. I cannot say exactly which bit. I don't want to advise one that is too wide or too narrow. You should be able to determine the right one when you go shopping.