D.L.
I had this happen once in my home daycare. A little girl I watched had it and the dad said it HAD to have come from my house. He was being pretty snotty but I checked my 2 children, myself and my husband. Sure enough, my daughter had lice! I spent the next 24 hrs cleaning every nook and cranny of my house to make sure I had gotten all areas. No one could come to my house for an additional 2 days to ensure there was no recontamination. I now routinely check all children who come to my home (even teens! LOL!) periodically for head lice.
Since I was a cosmotologist, I knew that I could use mayonaise instead of those expensive kits for the follow-ups. You could use mayo period but I always liked to be sure and used the Rid first. I used the mayo like a deep conditioner that you heavely covered the entire head and hair (nice and thick layer) then covered the head with plastic wrap as tight as your child can stand it. After an hour, I rinse the hair well and use the Rid comb. Then I would rinse again and shampoo like usual. The mayo smothers the lice and is so oily the eggs slide right off.
But if the daycare provider is not treating each member of her family, plus pets, plus mattresses, sofas, chairs, carpets, every towel, sheet, blanket, clothes and stuffed animal; the lice will return. She's obviously either not treating something/someone or another child's parents are not treating their child/home before coming back to daycare.
Did you know that lice can not attached itself to dirty hair? It's true! I got this straight from my doctor. Only clean hair/children get head lice.