I think there must be a culture clash here, as it sounds like leaving children unattended is expected in some parts, and it's probably fine if it's expected and that's the way things are set up.
I think the issue comes when someone behaves like that in a culture in which others aren't doing it or expecting others to do it.
Also, it's probably only an issue at between 3-5/ 6 years?
I wonder what would happen if there was an accident in that situation?
I absolutely would not take responsibility for any accident a child had because a parent decided to 'dump and run'. I would feel terrible but it wouldn't be anything to do with me. And I don't think most people I know would be ok with it either. If that parents decides to leave their child unattended, the blame lies squarely with themselves if the child goes missing / has an accident etc.
I was horrified last year as it turned out a child was 'dumped' at Ds party, but the parent only told one person, who was just another guest, and who was quite confused about it (as it's such unexpected behaviour to leave a 3-4 yr old!), so they didn't think to pass on the message. So no one actually knew there was a pre- schooler left without anyone looking after him! The child was self contained and didn't aak for help throughout the party, and must have taken themselves down two corridors and to the loo which was shared with a public cafe... Just because the child was ok doesn't mean to say the situation was ok!
Parties are not sealed environments like schools or daycare. They don't have roll calls and counting heads and vetted staff. There is no one there who's job it is to look after all children's safety and security.
There are people who are focused on the party itself, set up, games, food etc, and they are usually very very busy, and lucky to grab a minute just to sit down and chat to a friend. Then there are entertainers who are focused on, well, entertaining the children, not taking care of them. And there are guests who are there to have a chat and keep an eye on their own kids, and who are pretty oblivious to everything else. Basically there's a level of benign neglect which only happens/ works because each child is there with an adult ... even if that's a friend who's brought their kids and their friends kids, so they know that they are definitely 'in charge' of those children.
Parties are expensive and a lot of time, effort and preparation goes into them, without making them responsible for the welfare of every child in the place too. In the party culture I live in, it would be considered selfish to make these already frazzled/ run off their feet people also be unpaid child minders, or force them to have to pay for babysitters / extra people to take on that role. It would also make throwing a party unfeasible for many people, single parents without a huge number of friends and relatives for example...
I'm organising a party now for 4-5 yr olds, and I certainly wouldn't be expecting to be made into a glorified babysitter by parents who've never met me, never been to the venue and had no idea how the party is going to be run!
I would feel forced to say yes if someone came in and 'asked' me on the day if I'd look after their child as they were going to be left on their own. You can't really say no on the day when the child is at the party seeing her/ his friends have a lovely time in front of her. I wouldn't be happy though and would have to ask others to keep an eye on the child as I wouldn't have the time, and once it had happened once, other people would think twice about inviting that child again, knowing the parent was going to dump the child on other people with no notice or consideration...
Very different from a culture where it's accepted and a reciprocal arrangement!