I don't know that anyone can really tell you if it'll be worth it for you and your family, particularly considering the picky eater piece.
I would suggest you sit down, take a look at the restaurants where you believe everyone will find something to eat, and figure out the cost for your family. If you go to
http://allearsnet.com/ and look at their dining section and menus, you may get a better idea. Personally, I'd probably try to plan numerous buffets so that everyone can find something to eat. However, depending on the severity of your children's autism, this may not be conducive to their needs. Only you can really tell what will and won't work for your family. If your kids like to graze frequently then counter services might be better suited and wouldn't be the best value under the plan. If they really need the structure of some sit-down meals, then the dining plan may be better.
What I did for our upcoming vacation (given it's only two of us) is made a list of the restaurants where we want to eat. Then I pulled the prices from the menu and added them all up (including tax). Once I did that, I figured out where we'd probably eat CS and considered that we typically split our CS meals since there was both an entree and a dessert. I built some estimated CS costs as well as an average snack cost (I figured $3.50 per snack). We still ended up saving a bit of money even after I accounted for the cost of tipping.
As for reservations, I would strongly encourage you to go ahead and get reservations at the table service restaurants where you want to dine, particularly since you are going in one of the busier seasons. You 'might' be able to show up a bit early and get seated if your children get really hungry. The other thing you might be able to do is to buy them a snack (using the numerous snack credits) to tide them over until your reservation.