I never had to host a child with allergies, as there were none in my kids classes in primary. However the whole school was nut free as there was a child / children in other classes.
They are now in high school and there’s a different pupil with a severe air borne allergy so it’s also nut free.
I have hosted children who were lactose intolerant and gluten intolerant. Each time the parents offered to send lunch boxes. I accepted if it was a sandwich type party tea.
When it was a hot meal I sourced suitable products and checked in advance with the parent that it was ok. I felt it was unfair for the child to have sandwiches when everyone else had eg chicken nuggets, pizza and ice cream.
None of the other children at the party batted an eyelid because they were all in the same class to completely used to it.
I don’t know how I’d feel about being asked to use an epipen when I’m not trained. I suspect I’d ask the parent if they would like to stay and be a party helper, if they were happy to do that.
I’d never dream of excluding the child and would work the other parent to find a solution we were both happy with.
At what age do children start to use the pen themselves ? I know that children with type 1 diabetes do this from quite young.