Oh, you will all have a great time! Here are a few more suggestions:
If you have the space, take bubbles, balls, beach toys, sidewalk chalk, riding toys and gear. Lots of places to ride bikes and trikes and scooters.
Take a picnic to the beach and spend several hours there.
Take lots of snacks from home, and on your MK day let everyone fill a ziploc bag to carry in fanny packs. Then everyone can graze through the park and they won't be whining at the popcorn stands. You can even carry some bags of popped popcorn. Carry a small softsided cooler with some frozen juice boxes (if you can freeze them at the Fort). Keep everyone fed and hydrated--it makes the day go so much more smoothly.
On the MK day I'd pack a picnic lunch for everyone so you don't have to head back to the Fort. Normally I like to go back for lunch and a rest, but with only one day you may want to stay all day. Take a stroller for your little one and hang the food on the stroller. Peanut butter sandwiches are safe, and cheese and crackers and other less perishable things would be safe. Take fruit and anything else your crew will eat.
Back at the Fort, there are playgrounds in several areas. We always had to hit every playground every day--looking for new friends. Riding the bus is fun, too, and it is cool in the middle of the day.
If people get tired of the Fort (sacrilege, I know) you can ride to Downtown Disney to walk around there. The
Lego store has play stations and a playground (last time I was there, anyway) and if you want to travel more you can catch a bus to another resort and check it out.
You can also check out the Lodge and the Contemporary. Several times at the Contemporary they have had kids activities going on. One time Lauren got to make ice cream there. We told them we were from the Fort and it was fine for her to join in.
You can cool off in the midday by riding a boat to the Contemporary and riding the monorail for a while, too.
Don't be afraid to think outside of the box and give things a try. You'll have a blast!