G.T.
You might want to use an antibacterial soap like Dial.. our BO comes from bacteria. If you dilute a little bit of bleach in some water and swipe it under her arms with a cloth this may help as well. Since she's only 5 and probably doing her own bath these days, she may just not being doing the greatest job at washing herself. Make sure to use a wash cloth, lathered up and lightly scrubbing herself. Also buy her some powder that comes with a puff and have her powder herself before getting dressed.
5yrs old is pretty young for hormones to be kicking in.... but I've see a lot more of that lately, I think the human species is evolving somewhat and we are all noticing it in small doses.