For me, I think it would come down to the severity. My oldest son has a nut allergy. We have been prescribed an epipen, but have never even come close to requiring it, THANK GOD and touch wood. Last year, one of my school age kids was found to have a "tree nut" allergy. He was also given an epipen. I was not going to terminate care for them, though.
If a parent came and said "my child has a SEVERE allergy to such-and-such. He/she has been hospitalized, we have used the epi in the past, and even trace amounts can cause a reaction...." Eeeek. Too much for me, I think. I would also rather that child be in a group care scenario, as opposed to home daycare. More children, true. But also more adult eyes and hands.
I wonder if you could google "waiver document" and find an all purpose one that you can amend to suit your needs? Something stating that the provider will make all efforts to ensure an environment safe from the allergen, but in the event that something should happen, it is agreed that the provider WILL not be held personally responsible or liable.