You've had some good suggestions so far, and I have a few to add. We have a similar situation with our daughter.
If you're concerned, maybe you could check with your daughter's pediatrician. He/she may be able to provide you with some advice on how to handle the situation.
Also, you might want to try using a different bath soap. You didn't mention what you use, but I know that my daughter, who is 5 1/2, LOVES bubble bathes, scented kids soaps, etc., but these aren't always effective in dealing with body odor issues. What I do is use Dial bath soap (just a regular bar of soap) first to get her squeaky clean, then she gets to use her "fun" soap to her hearts desire!
If you are using something like bubble baths or scented kid soaps or the like, try using Dial, Dove, or something similar first when she hops in the bath, and then let her use her "fun" soap. This way she gets the oils off her body that can cause the body odor, and she gets to have fun in the bath with her soap as well.
Hope this helps. Good luck! :)