Hello, Colleen!
What a wonderful thing you're doing for your nephews and niece! They will have such a wonderful time at Walt Disney World, and you will, too, watching them all have such a great time.
I am so glad that you opted for the
Dining Plan! I have one Foodie child, and one picky-eater child (working on that one) and have found that the Dining Plan has really helped us introduced new foods to the kids when they are ready for it, and have accommodated our kids' culinary interests when we needed it to do so.
Now, the 6-year old will order from the kids' menu, so you will be sure to find items he / she enjoys no matter where you dine. The older three will be ordering from the adult menu, but I can tell you that you only need to talk to your server in order to ensure that each child's order is exactly what he'd / she'd want. If there is a sauce somebody doesn't like, see if there is one elsewhere on the menu that would be better. If there is a side somebody doesn't care for, look at the other sides, from other options, and ask if you can swap out sides from elsewhere on the menu.
Servers and chefs are always so happy to accommodate dining requests, so don't worry about picky eaters. On that note, head over to my favorite unofficial source of Disney information, allears.net, which posts menus from WDW restaurants. This will help you see what might be on the menus, and will enable you to book dining reservations at locations which have menu items you think the kids will like. Keep in mind, too, that buffets are a great choice when you've got a mix of folks who are picky eaters as well as those who really would like to sample more interesting food.
Send more questions as you get them! I love to help!