S.G.
My little 5 1/2 year old has the same thing. I asked the pediatrician and she said its probably nothing, there are no signs of puberty and not to use deodorant. For now, I dress her on the cool side so she does not sweat (mostly tee-shirts, light dresses) and always have wipes on hand to clean her armpits if she does get warm. Good luck and let me know what else you hear. Right now, I'm just trying to stay ahead of it.