D.N.
I always include a note in the invitation that they are welcome to drop the child off but please introduce themselves and provide a phone number so they can be reached in an emergency. I have never had any complain about it. I would rather meet the parents and know who is picking the child up. You never know what can happen. I had someone tell me taht was tacky but the parents all coorperated. I don;t think safety is tacky. It wasn't like I told them to buy a gift.