I totally get why you are appalled! Yes, as another person noted -- you have it on the forms, also ask now for a meeting with every teacher or aide or parent volunteer who might be giving out food at any point, and go over the action plan for if he is accidentally given PB or if he inadvertently grabs another child's snack and has a bite. I think meeting with them in person would bring home the seriousness of it. If they are trained professional day care providers (and I sure hope they are) they are well aware and probably absolutely horrified that it happened.
I also would dig into this a bit: Was there a volunteer there helping that day who would not have known? Was there a new aide in the room? Was something going on that day just before or during snack time that distracted the teacher/aide/volunteer (such as kid having a meltdown at that moment, or the teacher was dealing with a child who had an issue and the aide gave snack if she normally does not, etc.)
Whatever you do, approach it calmly and not in an accusatory way or they might get defensive -- and defensive people do not LEARN well, and they need to learn from this. When you meet with them, present it as "This time things got serious and then turned out fine, but we feel we need to go over, in person and all together, the exact steps. And you need to know that he may not immediately have trouble breathing - but it is coming" and so on.They may have thought that if he wasn't having trouble breathing he was OK and you know better.And next time, the breathing problem could hit much faster -- there is no way to know in advance.
Does the day care have signs around that warn "Peanut allergy --NO nut snacks for Jason, Sally and Bobby" or something like that? At the preschool at our church there is a big sign outside each classroom if there are children in that class who have allergies. Even our elementary school has such signs at every classroom door as a constant reminder. If the day care has a food prep or food serving area, I would want signs up there as well.
Many schools and preschools -- even many camps in the summer, afterschool programs and even my child's DANCE school -- now just ban peanut products, period, forever. Because the allergy can be so very serious and hit so fast, that seems like a wise course particularly in a day care or preschool setting, where the children are young enough that they won't think to say, "Does that have peanut butter in it? Has that cookie my friend brought in for his birthday celebration have any nuts?" and so on. Older kids can get very good at policing their own food but for these younger kids, I would discuss with the preschool the idea of banning peanut products. A truly allergic child could end up sick just because a friend was a messy eater and didn't wash hands enough after a snack time.
Be aware that some other parent is going to fuss and fume and say, "MY child can't have his favorite snack because of ONE kid, and that's unfair!" but to me that's boo-hooing and whining; her child can have his PB at home any time and all the time. Offer ideas for other protein snacks that kids like, such as cheese, in place of PB.
My child does not have this allergy but I get why it must be taken extremely seriously. You don't want to explode at the day care folks -- but you also are going to have some real issues trusting them now. Make sure they can see that and tell them you expect them to work with you because your son ended up in the ER directly due to their mistake, and you need it to be a learning experience for them -- not just for his sake but for any kid with a serious allergy.