Okay, I only read the first page of the thread ... are you serious? She's five!
If it helps, my parents never even offered me a swimsuit when I was five. I wore the same swimwear as my brother, and I started to cover my breasts when they, you know, actually started to grow.
I did not grow up to become a boy. In fact, I became a radical feminist, and am about as far from being transgender as one can be.
Have you ever considered the danger that your daughter might consider herself male exactly because as a girl, she cannot wear sensible clothes?
Removing a one-piece swimsuit to go to the toilet is rather difficult for a child. Forcing your daughter to do that will teach her at an early age that to be a girl is to suffer. Do you want her to learn that?
Do you want her to hate her female sex?
If it makes you feel more comfortable, maybe you can compromise on a bikini bottom with no top, though I have to say I consider bikinis for girls of that age a perversion and would not, personally, contribute to their continued existence.