I can tell from reading the previous responses that this post is not going to go over well, but I'm pretty flame resistant, so here goes....
It is very hard to tell what arrangement had been discussed between parents before reservations were made, but I doubt that anyone would book a 15 day trip without having checked that schedules were/should be clear, whether that means talking with Mom or already having visitation scheduled. Either way, it doesn't sound as if the mother is being very accommodating. Her ex-husband is letting her know almost 1 year in advance that he wishes to take the child to WDW for 15 days. Her birthday overlaps the trip by 3 days, either at the beginning or at the end. It is difficult for me to imagine that she can not bring herself to celebrate a few days earlier or later so that her child can spend 3 more days at WDW. Obviously, the trip dates can't be shifted as the trip is being shortened, so it is probably based around work vacation times, which are not apparently flexible in this case.
I'll have to be one of the first to say that this mother sounds very selfish to me. I agree that birthdays are a big deal, but in the vast majority of cases a 15 day trip to WDW is a HUGE deal. If this mom were truly putting her child's needs and desires above her own, I seriously doubt that she would be shortening the child's trip.
I'll also add that divorce sucks, no matter how you look at it, but parents really need to be grown-ups and put the children first. For those of you that would deny your child a trip to WDW with their other parent because you wouldn't want to miss it, SHAME ON YOU! Divorce usually means that one parent or the other is going to miss out on some things. That is the nature of the beast, but to deny your child something so wonderful as their first trip to WDW because you are not the parent taking them is just wrong. How would you feel if the shoe were on the other foot, and your ex was saying the same things that you are?
[Stepping off my soapbox now and frantically searching for a nearby fire extinguisher...

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To the OP, I'm sorry this is happening to your family, but I think that you may need to prepare yourself that this won't be the last time something like this will happen. Enjoy the 12 days you have, and though it may be very hard, remember to not say anything negatively about the mother in front of the child. Someone has to be the grown-up!!
