Our 6 year old daughter goes to parties all the time.  We do not tell the parents about her condition (unless they already know).  If she is showing any signs of being unwell we would not send her and we always leave a number for them to contact us if she’s not well, we would only ever be about 10 minutes away at most.   We certainly wouldn’t expect strangers or people we don’t know very well to inject our child, that would be an extraordinary request and I would imagine that this information would circulate around to other parents and perhaps your child would be excluded from other parties because of it.  Parties generally go for a couple of hours and as long as the parents are responsible enough to contact you if your child is unwell that I think that would be enough.