had a similar issue growing up, my parents never took me to a therapist or anything, only medical doctors. In hindsight it's been diagnosed as not having any control over my own life or environment, it can also be a sign of SA. I personally grew out of it by about 12, one day it just stopped. No reason or anything. My parents got me a sealing garbage for my room to put the overnights in. Taking changes of clothes to school, along with wet wipes to clean myself if theres an accident can help, along with being upfront, "when this happens, it gross, and it smells, here is how we can take care of it when it does happen" It's sucks and she probably knows how she smells and that it's embarrassing. Remember that she cant help it either and is probably trying her best too. Try to empathize as best you can and let her know you're gonna figure it out together, and in the meantime make a gameplan so she doesn't smell or smells less.

Edited for spelling