If it were one of my girls I'd just sit them down and explain to them that I knew.
husband also bought each of my girls a single red rose to mark their transition to "womanhood". lol The girls thought it was really dorky at the time, but now recall it as just about the sweetest thing their Dad ever did for them.
This is pretty much a no win situation. I'd tell her, then explain about proper hygiene during this time of the month and why she shouldn't go swimming in a pool wearing a pad. There are deordorant sprays for this, but I'll warn you it can cause infection if used improperly or too often. You can find them with the douches and such. Usually good hygiene will get rid of the odor. On occasions when I had to wear pads I'd shower really well at night, then take a quick bath in the morning for an extra cleaning.
((hugs))
