Thanks for your thoughts everyone.
TBH, I'm not keen to ask the friends to come over earlier. I know their kids have activities earlier in the morning. We have invited them for lunch and the afternoon.....so that's what I think we should deliver. The party is straight after lunchtime, so it would really interrupt the day - the children wouldn't really get any playing together in, plus an adult would have to take DD to party, leaving the guests for 30 mins when taking her, plus another 30 mins at pick up time.
If our guests didn't have stuff on earlier in the day, or if the party was later in the afternoon, I might have thought about your suggestions to ask them to come earlier - however, I would do it reluctantly, because I don't like to mess around with plans too much once they are made, because you don't know what other stuff people might have arranged in the meantime and how the new plan might interfere.
Yes ammiciss - the friends certainly agreed to come over, on the basis that us and DD would be around - adult company for adults, child company for children and a bit of us all together during the meal. If they had known DD wouldn't be around for most of it, that date wouldn't have even been considered, because it would cease to be an event with company and enjoyment for all - so DD not being there would change the whole dynamic. And yes, if DDs party invite had come first, we would have said yes and that then would have been the event we stuck with - wouldn't have considered a day with our friends, as DD was already committed elsewhere.
Sirfred - it's not that kids plans are less important at all. In this case, the thing she would have liked most came up AFTER alternative plans (which happened to have been made by me, after co versatile with DH and DD) had been made. If her party invite had come first, that would have had priority and I would have been prepared to turn down other invitations whether they were for adults or children. In fact, we have often done that because DD has been invited to things before invitations have come for us or the whole family. It really isn't about adult activities or choice having priority, but committing to something and sticking to it.
And regarding making plans for the family=being in command, rather than it being consensual - well, in lots of things, as the adult and parent of a small child, I accept that yes, I am in command, if you want to use that word. It is up to me when DD goes to bed and if I think social events are suitable. And it is up to me to decide if the practical arrangements for social things she would like to do are practical for the family....because she is a child and can't be expected to make judgements about logistics and bigger picture stuff.
This said, family arrangements are not 'forced' on the family. So DH and I might have a conversation about inviting people over and some possible dates and we may well ask DD if she would like to see those friends. It is then likely to me who has the phone conversation with other family and pins down the time and date. Yes, I then do tell DH and DD, but this hasn't been forced on them at all. There hasn't been any arranging for the family to show I'm in command, or lack of consent - we are all in agreement that spending a day with this family (or other activities we choose to do) is what we want to do. I certainly don't decide the plans for the whole weekend and just go ahead and schedule everything in without conversations. Often DH has the idea of what to do or DD will or we will chat about a few things we would like to fit in over the next couple of months. Someone has to make the calls though and fix the dates and times....and I can only do that based on info about other things I know are going on, at that point. It's not possible to predict what other options and invitations might later appear....but we accept that if it involves other people, we stick with the plans first made....and sometimes we or DD don't get to do the thing that appears later and looks more fun.
And of course, with things that don't involve other people, we can be much more flexible. So if I was planning to visit the shops (Sirfreds eg) and an invitation for party turned up, we would certainly shelve the shopping. It's only things that involve others where we try to stick with the original plan.