It's just something kids do!! My son is 2 and will take a box of frozen waffles and lick the picture on it!! Nasty??Yes!! Do I make a big stink out of it?? NO!! Because it will only make him want to keep doing it and then he'll reach for other stuff in the cart to lick. I just calmly tell him we don't do that and after a few more licks he stops because I didn't feed into him with my reaction. Your daughter is going into pre k, she is going to get other peoples germs, new and old. All you can do is make sure you are teaching her proper hand washing (after the bathroom, before meals, after coughing/sneezing if not doing it in the side of her arm, after playing outside, etc) and hope the school she will go to uses proper cleaning techniques for the toys and what not. In the mean time, just ask her not to put her hands in her mouth without making a big deal out of it. You could also tell her since she puts a blanket in her mouth too that we only put food in our mouth and when you see her with anything that isn't food, remind her by saying Daughter, is that food? She answers no and then you say remember, we only put food in our mouthes!!
Good luck
S.