OP, we will be there at the same time as you.
My kids bring their own money that they have saved. We give Disney dollars for extra chores, etc. We save all our change as do they. They save from the time we get home from a trip until the next one. Their money is theirs to do as they wish, and they are very frugal that way. I usually but them what I wan them to have, like a new hoodie. Also, six years ago my DD9 (then 3) wanted the Cinderella's Castle playset. It's $75. She didn't have enough to buy it but I didn't want to buy it "just because". I got it and hid it and Santa gave it to her. I had told them on the trip to behave cause Santa would be watching and when she opened it, she said "Santa was watching and he knew I really wanted this". We still have that playset set up in her room. You are going at a perfect time for Santa to do a little shopping. When we go back my kids still say "Santa's watching", it helps them behave when they get tired and cranky. I always do some of my Christmas shopping when I am there, even stocking stuffers are great from there.
Last trip my DD9, then 7 wanted to spend $50 of her own money to buy her little sister an ariel snowglobe. I tried to talk her out of it (that was a big chunk of her money), she said it was hers she could do what she wnated, and she was right, she had saved that money. She gave it to her sister right away who LOVED it, and then wanted me to take just her shopping (she was 4) to buy her sister something. She bought her a Pluto stuffed head that sings and lights up a red nose like Rudolph. When thye spend their own money they feel good about it and make really good choices. Those 2 items are some of the best we've ever bought in disney, both as an item and because of the momeory involved.
So, start today, maybe a good behaivor chart. Give a set dollar amount and by the time they get there they will have earned their spending money. You have 33 days till you go (I know, i'm arriving the same day,) they could have $33 and maybe you could bounce it up to $40 if they earn a dollar every day as a bonus to make sure they work hard every day (or do whatever $$ amount you decide.)
Have a great trip, and I hope you have great weather!!!
I forgot to add, my kids like to look all week and make a list, then decide at the end of the week what they really wanted, it makes for less disapointment of "why did I waste my money on this?"