Sandandsea, I liked what you said about the key issue being to teach children how to deal with situations they find themselves in.
The answer cannot always be 'ring mummy'. Children need to learn to assess situations and to decide if something happening is just something they don't like that much or something which needs adult intervention. They need to learn how to tell a teacher or the adult in charge - it might not be easy, but it's really important and something parents neglect to do at their peril. As parents, even with phones and the Internet we are not here 24/7 and children must learn to turn to others too - that is so important. Very small children cannot do this - that's why they aren't given much independnedence and don't go to sleepovers.
Parents have to make judgements about what their child is ready for. Just because other 6 year olds go to sleepovers, doesn't mean they are all ready. If your child won't manage a night away from contact with mum or dad, or in liklihood want to come home in the middle of the night, just wait a while until they can cope better. A phone isn't a substitute for having the skills to be able to handle a situation and a phone shouldn't delay parents teaching their child those skills.
I wonder if parents are giving their young children phones in the mistaken belief that they have somehow covered them for all risks - that now, they do t have to teach their child to communicate with the world, or to assess situations or to make judgements about what is safe and not.
And I think it's important that if parents send their children away overnight, whether with the school or with other parents, they understand that in that place, the other adults rules will apply. That's also something to teach your children - whether away on play dates or overnight. Surely no-one would say that because at home the family eats at 6, expecting a visitor to eat at 7 is unacceptable, or because a child goes to bed at 10 at home, asking them to go to bed at 9 whilst on a sleepover cannot be acceptable because it goes against what the parents have always done, or because the child has the TV on all the time at home, they must be allowed it all the time whilst at a play date, makes sense.
When we let our children go elsewhere, we do handover responsibility for that short time. We expect the other adults to look after our child and we trust them to do this, knowing it might not be in quite the same way we would. Most people can cope with that and learning not everyone does everything the same is part of venturing out of the home into the wider world.
Parents who want the same rules as they use at home used in every other home, especially when they have never even bothered to make clear what those are, are just not in the real world and are self-indulgent and creating a narrow little bubble for their child, which doesn't foster gradual and growing independnetnce and venturing into the world.
If you have a particular thing that as a family you do and feel is really important for your child if they are staying elsewhere, THEN MENTION IT TO HOSTS AND ASK HOW THEY FEEL ABOUT IT - notice I don't say, just inform them, but have the courtesy to ask them, so that they can say if they are happy about it or not. And if they aren't, and it's a deal breaker for you, then the sleepover doesn't have to happen. Simples! But don't just assume they will do things the same way that you do.