As parents picked up their kids from my grandkids b-day party, one?
I don't think that's part of the deal for being host at a birthday party. You're supposed to feed them, ensure their safety and make sure they're enjoying themselves. If guests have special needs, you have to ensure these are catered for. Obviously, if the child was falling over because they couldn't see through their glasses, obviously you would be negligent. Otherwise, that's like blaming you for a little girl getting her dress dirty.