Rachelle - congrats on planning your first trip! No matter what you end up doing, you are likely to find that your older kids will most definitely find attractions they absolutely love. Walt Disney World has a variety of thrill rides, from The Rock N' Roller Coaster Starring Aerosmith and The Tower of Terror at Disney's Hollywood Studios to Expedition Everest at the Animal Kingdom that many older children enjoy.
As for packages, I suggest you visit the
Disney World Specials page to see what special offers may apply to you. Consider purchasing a dining plan, as it's a great way to save money and know what you are spending prior to your arrival.
For accommodations, check out this section of the
Walt Disney World web site which contains information on rooms that can fit 5 or more. You may want to go with two rooms, or one villa.
Finally, check out this post
HERE which provides some tips on how to travel on a budget.
Have fun!