Both of these trips sound like fun!
Forvdrogerstn, I would start with ordering the Disney planner. Watch the video and get excited!
Start planning where you want to stay, what the kids want to do, and where they want to eat. Get everyone involved! Assign one person to different tasks, getting info on costs for air, hotel, food, passes, etc. Introduce them to the boards, and wdwig, and any other site you can think of.
Talk about any issues they may encounter for being under 21. How to deal with it and emergencies. Make sure SOMEONE has a credit card that can be used for an emergency (medical visit, car repair if they drive, etc)
Meet every other week or so and get updates and plan the budget. Once the budget is planned, then have frank talks about how they will pay for everything both now and at Disney. Disney dollars are great gifts., If money is tight for anyone, encourage part time jobs or even od jobs around the neighborhood or amongst the families. Have a spring tag sale to help pay for it.
Above all, I hope they have a wonderful, safe trip!