Our DD has had pongy pits off and on since ahe was around 8? She's 12 now.
It seemed to be worse the day after a garlicky meal and if she wore some tops that weren't cotton. We just ensured she washed herself really well at night and did a wipe of her pits again in the morning before getting dressed.
We even looked into some of those natural mineral deoderant at one point, but waited until she was 10 before getting her some of the teen sprays.
Now she's a bit older if seems to have lessened. http://www.livestrong.com/article/74538-body-odor-children/
It wouldn't hurt to ask your GP when you see them again.