Why is everyone so mad?! OF COURSE kids share germs, and this is just the beginning of it. Wait until elementary school and LICE is running rampant.
I would call the pediatrician. I'm not sure if little one's can use Abreva, but that's supposed to be the best stuff ever.
Going forward, teach your daughter not to share chapstick, drinks, hats (for the lice thing, LOL), etc... and give her her own little thing of Purell (if she's school age) to tote around. Make sure she's big on hand washing.
I'm a huge germaphobe, but I totally understand how kids LOVE to share germs. Education is the key in preventative measures.
Also, be sure it's a cold sore. I don't remember if I've ever had one as a kid, but I've never had one as an adult. People think 'herpes' and freak out. Cold sores, or herpes simplex as everyone else mentioned, are totally manageable and WAY different from the herpes most people think of (vaginal herpes). Talk to her doc :)