I have not read through all of the responses, but seeing how I have a 12 yr old Stepson i'll put my 2 cents in!!!
We just went to Disney world and He and my 3 dds had banks that were for there Disney spending money. My girls did great, but he kept dipping into his. Dh just told him that if he uses his spending money on other things then he will not be able to buy himself any souvies. The trip itself was being paid completely by us of course and everything we did like mini gold and renting boats and surrey bikes was all paid by us too. Their money was for what they wanted to buy.
A few months before the trip he kept taking some money out. But once it got closer he started seeing the light at the end of the tunnel and ended up being very good with the money.
Next summer is too far away to tell an 8 and 12 yr old not to buy ice cream or swim because of the trip next year. They live for the moment, as they should.
Let them be kids
And one more thing, you may or may not want to hear! But because I know sometimes it's tough with stepkids, i'll say it.
If they bring money from their Mom's house, I wouldn't tell them to put that away for your family trip, if they bring that, they should be able to spend it as they wish , and trust me, once the trip is closer they'll be glad to put their money away for it.
I would go with allowance at your house for helping around the house. Set a dollar amount every week, but don't hand it to them, put that money away for their spending money.
And don't cancel the trip, your dd will miss out too! If they don't save their money, they won't buy souveniers !
We just came back from our first trip a few weeks ago. I had my kids start saving up right before Christmas, we're going back next year, and I won't even start the saving that far away, they just were not into it, it was too far away. I'll probably start 3 months out, they can use holiday and bday money.
Good luck