Assuming you are tent camping at the group campsites - Bring things (balls (volleyball, basketball), frisbees, bubbles, crafts like tie-dye tshirts etc) for them to do at the campsite while you are cooking/relaxing at camp. The group camping area is really nice and big for them to have fun! The firepit setup in the group campground is ideal. If you can bring their own bikes, they would love to be able to ride them around. Wagon ride, campfire singalong, swimming pool. You can also look at some of the GS badge guides - it's the perfect setting to do some of the Brownie badges! I don't know about you, but it isn't very often that I have the girls in an outdoor setting together to tackle some of those badges! One note - Try not to stress out about getting a lot scheduled for them to do. Getting from place to place takes time at FW, especially with a group. So, make it fun and give yourself plenty of time for transportation. That will be part of the fun anyway! They will love walking around FW.
You can get takeaway food from Trails End Restaurant at FW. If you want to eat together at a sit-down meal, Trails End is a really yummy and affordable buffet for breakfast or lunch. My 10 year-old and 4 year-old loved it! At the MK, Pinnochio's Restaurant overlooking It's a Small World is a favorite place for counter service. Also, Gaston's Tavern is a fun place to go have a snack.
Go easy on yourself for breakfasts. Get them yogurts, granola bars, cereal, etc. Also, if anyone has a mini-fridge, you can bring that to your campsite. We did in March for our family. (Again, I am assuming you are tent camping, not staying in the cabins.) Also, bring plenty of bottled water with you. I bought gallon jugs of water from Big Lots and then fill up water bottles to bring to the parks. Oh, the ice machine at the group campground only takes quarters, so bring quarters if you need ice and/or to use the laundry room.