If my child was going to Disney with her father, step-mother, and her siblings from that relationship, I would most likely not give her any spending money. She would be going with her father. If I, her mother, took dd to Disney, I would expect to fund the vacation fully, including spending money. Another reason that I would not give her money is because it would not be fair to her siblings that are going. It would not be fair that she would have more money to spend than her siblings (I would assume all the kids would get an equal amount of money?). I would also tell my dd to use the money her father gives her on whatever she wants to buy. I would not expect her to buy me anything. Now, let's say that I wanted a particular coffee mug from Disney, then yes, I would give her (or her father) a $20, say "Don't forget my Mickey Mug" and send her on her way.
Now if I took my child to Disney, gave her a set amount of spending money, and she decided to spend her money on a gift for a family member at home, I would be one proud Momma.
Being said, my answer is hypothetical.