OP- I know you're getting bashed quite a bit and there does seem to be some animosity towards the girls on your end. Maybe it's because you can't stand their mother or maybe they disrupted dreams of Disney with just your son or perhaps taking on two girls at that age is a bit harder than you imagined, whatever the case, your posts come off a bit harsh toward them.
In an earlier post of mine, you agreed with what I had to say. You probably wouldn't be getting so many people against you if you simply said "All kids are getting the same amount of money. They can spend it on what or who they choose, but when it's gone, it's gone. Along with their money, we will be buying each child a shirt/sweatshirt, a pair of Mickey ears and one toy." (As for your son having his own money, why not just put it into a savings account instead of spending it at Disney? He's young enough that he won't even notice/care if he gets something. Get him the same thing as the girls and maybe a stuffed animal. His money can alway be used at a later date when he's a bit older.)
You are merely speculating that the mother wants gifts brought back. As many other posters have said, gifts aren't necessary and if they did want something specific or a surprise, then money would be offered up. I'm going to make the assumption that the mother doesn't need a gift and the kids don't need some small trinket to remind them that they may never be able to go. Have the girls send them all a postcard (Should cost a whopping $5) and bring back some Mickey rice crispie treats for each one.
There are still a few months before your trip, so no need to worry so much about this now. Their mother probably hasn't given this trip much thought at all and most likely won't until a week or two beforehand. Go and have a good time and remember that Disney World is a place for creating memories and hopefully bringing your whole family closer together.
