The people at any fetish party should be accepting of your kink. (If they're not, find a more accommodating fetish party.)
I certainly have worn baby outfits and a diaper to such parties, and the worst reaction was vague amusement.
You have to remember, though, that at such a party you are expected to be a grown-up in a diaper, and not engage in age-play or similar. That means you have to stand around, in your diaper, and talk about football, politics and the weather, while sipping your beer/wine/coffee/whatever.
After a while, it all seems perfectly normal. Sometimes, too normal, since it is remarkably easy to forget that you're in a pink frilly dress talking to someone cased head to toe in latex.