Morning. Thanks everyone again for your thoughts. It's interesting to see the different views.
I think itsatiggerday is probably right in terms of understanding why people react differently to this Q. Many of our friends are old friends, some of whom live round here, but many don't. We might see some of them once or twice a year, for a family day together. Many have kids of a similar age to ours and we have known the adults since before we had kids. All of our diaries get pretty full months ahead, so finding dates to do things isn't easy and getting together can involve up to a couple of hours travel each way.
As I said up thread, I make the arrangements having discussed with DH and DD what we might like to do and who we might like to catch up with. They are family plans, made together. As I also said, I can see a time when DD is a teenager (and the kids of friends are teenagers too) when our get together soon might become a bit more flexible in terms of whether the kids will or won't be around, as they make more of their own arrangements....but for now,at 9, DDs social diary is still organised by me. She enjoys these gatherings with other families and has played with these kids since they were babies - I realise it's not the same as friends you see every day at school, but......she can see them every day, whereas she only sees these others twice a year. As others have said, we can arrange to do something else with the school friend.
So I have decided that we will definitely be going ahead with the original plan,as I have always thought. DD hasn't strongly argued for going to the party, just expressed a bit of regret that she won't be going and has asked if it will be possible to do both, which I said we would think about. Now I have decided.
For this particular case, I have decided that we honour the original plan, because it was made first. DD is still at an age where we make plans to do things as a family and I'm happy with that and don't see it as her being subjected to our whims and her opinions not mattering. We give priority to the family arrangement not because it was made by the adults, but because it was made first. If the party invitation had come first,new would have accepted and then if our family friends had invited us over, we would have turned the, down and not been flaky towards DDs friend. So first booking retains priority, unless in an extreme situation...and this certainly isn't an extreme situation.
DD will understand - because she's is actually pretty mature for a 9 year old. Although she will be a bit disappointed (in the same way adults feel a bit disappointed when they can't make a social event) she will also understand why we are doing what we are. She will also recognise that there have been times when we arranged for her to do stuff first and WE then missed out on something we were invited to later.....just how the cookie crumbles really. And I'm glad that I can say in confidence that this is how she will feel. No doubt some posters on this thread will find it hard to believe she will cope and not feel hugely let down, or that her wishes are not considered, or that adults steamroller over her wishes. But actually, because we have consistently followed this 'first arrangement has priority' approach, she actually understands it and knows it is the right thing, even if it's a bit disappointing on this occasion.
I will continue, it's both local and non local friends to try to stick with first arrangements. The posts of here make me more convinced that constant changing of plans to suit oneself, is both flaky and a bit selfish. I'm keen to be respectful of my friends and their time and to acknowledge that cancelling people or trying to change timings last minute to squeeze in things that have recently come on the horizon is putting me/ourselves above them - it would grate if people did it to me, when I have a busy schedule which is hard to adapt, so I know it isn't easy for them either, however much people are polite and say they don't mind.
Thanks again everyone.