This is often not serious, but can be a sign of an underlying disease process. These children should have a thorough physical and lab work, to be sure. It sounds like the situation at your daughter's daycare, and across the United States right now, is probably caused by one or more of the viruses that thrive during cold and flu season. If the hives last longer than about 2 weeks in any of your daughter's friends, I would look for other possible causes at the daycare. In the meantime, Benadryl and a little TLC should get her through.
Hives can be quite uncomfortable.
I remember getting hives after being stung by a jellyfish, and I couldn't sleep without Benadryl for several nights. I also discovered that distraction can sometimes help with the itching. This is one situation where videos and kid cassettes might be a real boon. And perhaps an extra hug, or two. This is not a substitute for medical care or your doctor's attention.