What do I say to my daycare centre employer?

I just started working as an afternoon teacher at a nearby daycare four days ago, but I'm already having second thoughts.

I've been given a 2yr-old class and it's okay, except that I'm totally uncomfortable with changing their nappies (diapers). The sight & smell makes me want to gag every time, and I don't think I can keep it up for the rest of the summer. I had never changed nappies before this job, and I honestly thought I'd adjust to the whole concept because when I see other college students changing them, they're happy and smiling and having regular conversations, etc... But it's been 3 days and I know this is something I won't adjust to.

So, I'm thinking about talking to my supervisor and asking her if I can possibly get an older class or resign from the job altogether. My ultimate question is how do I phrase this to her without sounding petty/wussy/unaccommodating/stupid? I'm really embarrassed to bring this up in conversation and have no idea how I'm going to start. Bear in mind that I've only had the job for 4 days (today's my fourth day, I want to speak with her about it today). Keeping the job and just dealing with it is not really an option, I'm hating every weekday because of this:(

Thanks for your help, and I'm sorry if I sound like an uncooperative pain in the butt to some people...
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Hey, tell her the same way you just told me. I think it's more than fair. You tried, but it's just not for you. When dealing with children it's very important that the right kind of person has the job. And it doesn't mean you're a bad person if its just not what you are cut out for. It's better to be honest than to hold on to anger, disgust, and resentment. It's not like you work in a factory, you work with children who can be sensitive to your feelings. It's not only your own needs that you have to worry about, but theirs.

If your supervisor won't put you in an older class and you have to resign, it won't be the worst thing in the world. At least you'll have had some experience in something you didn't like to do with the opportunity to leave and not be fired. That's a lot more than many people can say. Just be honest, good luck.
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Just tell her that you've never changed diapers before and you don't think it's something that you can continue to do. She probably won't understand because I bet it doesn't bother her but she should be sympathetic and willing to move you, provided you're a good employee.