If you have something special coming up like the steam train event, could you tell him as soon as you book it so that he knows that day is out of the equation before he starts thinking about dates?
I know this might give him a reason to say 'aw shucks I was going to choose that date and if you won't let me see the dd then, I'm not going to bother that fortnight' or 'no, I have something booked that I'm not going to compromise on' - all sorts of ways of him to twist it but at least then he won't be able to say that he didn't know.
THink you are being reasonable to let him have them if you have childcare booked and loosing out on it - hopefully they will let you change the date if not get it back! but definitely not unreasonable to stick out for the special date for steam train event.
Do you know if he has any similarly special event type dates that might come up in the next few months that he could give you special notice for so that he can see things from both sides? Even if he doesn't have exact shifts yet, he might say it's my birthday or my mum's birthday at the start of december, so I want the dd's to be involved in a celebration that first week of the month, I'm just waiting to hear shift details. Then you would know that if you were planning anything that week, rather than booking something in regardless, you could double check with him and he would see the benefit of the system?
DOn't want to make this sound like you should pander to him at all - just hope that it might see he can be adult about making arrrangements in advance even if he doesn't have an exact date.
Alternatively - do you think he knows that the steam train thing is coming up, that it involves your bf and he wants to screw the day up?
fingers crossed that you get to keep the girls that day and have a fantastic day!