I understand his reasoning behind changing the weekend (although I'm not quite sure what the coordinating part of it was if his ex is usually OK to have their children and your parents have agreed to have yours). But leaving that aside...he arranged a surprise weekend a few weeks ago to make up for it...but didn't tell you it was to make up for it until now.
I just find that a strange way to do things. Not strange in a bad way, just 'strange'. It's like you saying you'd love to go to see Ed Sheeran for your birthday, he couldn't get tickets and rather than say 'look love, they've sold out of Ed Sheeran tickets. Is there someone else you'd prefer to see' he books Olly Murs because 'they're similar' - both being male singers.
If you can't get what your partner wants, something they've specified - not just hinted at, surely you talk to them about it? Well we do!
My DH knows, for example, that as I was born in the depths of winter, don't trouble booking a restaurant. I hate going out in the cold. Book a favourite take away, find me a choice of films on Netfllix, spoil me rotten with pressies and I'm happy. He knows, because he's asked me in the past and done what I've asked, that I wouldn't be looking forward to a meal at the Ritz around my birthday - I want to be warm, snuggle up and enjoy my day in my jeans and jumper. That's me. Even my DS said, last birthday, 'I told dad not to book a meal because you wouldn't enjoy it - too cold eh mum?'
I'm sure he meant well but it's not even the whole family thing really is it? He took you away for a surprise but not to the cottage you'd set your heart on. It's that as well as the 'being alone together'. The previous weekend away wasn't anything to do with this in your eyes because you didn't know it was supposed to be.
So YANBU. He needs to get that cottage booked for the two of you even if it's in a few months time. He needs to ensure you do as little as possible on the family weekend and, when that cottage for two is booked, he needs to make you feel incredibly special. Because, good intentions or not, he ballsed up. And we try to make up for balls ups when we love someone.