First, do what you feel is best for you. If that means wearing a dress to the prom, then so be it. What I would say is that your parent's concerns, whether real or hiding a more hidden agenda of tempering your trans feelings, may have some validity to it. The truth is it will likely be a shock to your peers, if you usually present male or that's what they know you as, to come to the prom in a dress. I'm not sure of your situation, so I can't really tell the extent of everything. Of course, you don't deserve any backlash for such an innocuous thing, but I don't want to tell you everything will go smoothly when I really don't know.
Having said all that, this is your life and your prom. If going in a dress and being true to yourself is really important, does everyone else truly matter? Is any of the potential backlash you may receive more important than your own happiness that day? That's a question that's better answered by you as we all have different perspectives and comfort zones for our lives. Find yours and follow through accordingly. Good luck with everything and I hope your prom is special as well as memorable in a positive way.
* Edited to make more sense, lol.