Hi Jonathan! It's great to see you again here on the Disney Parks Moms Panel!
One of the things I love about Walt Disney World is the fact that they haven't overlooked any of the little details that make your vacation special. The first time I visited Disney's Caribbean Beach Resort, I brought along a little sand pail and shovel for my son to play with at the sandy beach areas of the resort. Imagine my surprise when his kid's meal came tucked inside a sand pail, complete with shovel! On our last visit there, I saved room in our suitcases and just let my son play with the sand pail that came with his meal. As you suggested, we like to visit the Old Port Royale Food Court for lunch on our arrival day, so we'll have the pail first thing. It amazes me sometimes how simple things can be so entertaining for little children, so I think the sand pails will keep them happily digging without your having to load up any extra toys in your luggage.
When packing for my little guy, I like to bring two pairs of swim trunks for him so he always has a (relatively) dry pair to wear. Also, don't forget swim diapers for the baby, and more if your 3-year-old is still training. All those diapers can take up so much space, so I used to put them in gallon-size Ziploc bags and squeeze the air out to flatten them as much as I could. It's all about saving space in your luggage!
I hope this helps your family have a fabulous time at Walt Disney World, Jonathan. Have a magical day!
Kathy K.