Two things came to mind - (actually three, four) okay -
First - CHANGE her toothpaste/mouth wash. I am SO highly allergic to Crest that when I used it as a kid? My feet would break out in HUGE lumps so bad that I couldn't walk - No rhyme or reason there - but something in Crest made my feet break out.
Second - Rosacea sounds like something JUST for adults - but kids can get it too -
http://www.livestrong.com/article/328293-rash-like-pimples-on-the-face/
Third - Angiodemas -
Fourth - Allergies to laundry detergent - and this is going to sound bizzare - but IF she's allergic to your laundry detergent - and she has allergies, sleeps with her mouth open....the drool from sleeping with her mouth open keeps her mouth moist at night, moist mouth against the pillows or stuffed animals COULD make her break out?
If she has LONG hair or hair long enough for her to pull around to her mouth - twirls it, or dampens it and chews on it (not that it's a complete habit) but occasionally does it? Does she have a perm? Could it be the perm chemicals?
Throwing out for consideration.
If it is any of the aboves - you do NOT want to put ANYTHING on the outbreak - because the moisture from most of the creams and topicals would only SEAL more moisture in.
Personally? I would get her to a dermatologist. Call one - and say - I have this kid, has this problem it's intermittent - can I have an open appointment? Then take her when it happens again?
She may just need Benadryl, or an antihistamine. Or Tide Free, or SULFATE free shampoo. Or Some new kind of toothpaste.
I dunno.
Hugs and quacks to the duck.