What a sweet aunt (and sister)! What is your overall (TOTAL) budget goal? A few sugestions...
1. Stay at Bonnet Creek in a 2BR/2BA deluxe. It is directly next door to Caribbean Beach Resort and has a free shuttle to the parks. The villas have full kitchens and laundry rooms in each suite. This allows 2 things - less luggage, therefore less airline baggage costs, and very little in food costs. Amazing pools, slides, lazy river, free minigolf, etc. You can probably get this for less than $100 per night.
2. Lower food costs by using your villa kitchen for almost each meal. If you must do a character meal, make sure it is at breakfast when the cost is so much lower. If not...yea!... you just saved moolah. Buy cheap and easy food to prepare, bfast in your suite, lunch either in your suite when taking mid-day breaks or packed picnic-style for the parks and dinners again in the suite. Consider the 99cent certificates for kids meals at TRex and RC in AK.
3. Do the YES Program!!
4. Fly.
5. Either let each kid earn their spending moneyor give them each a $25 gift card to use on a souvenir (cost will be capped and equal, no haggling).
Keep us posted!