We got ours "free" and I don't think I'd do it again. Too many desserts, too many snacks (unless you use them up at F&W - that would be worth it). Then again, I might consider it again, but use it differently. Instead of dessert, choose something that can be eaten for breakfast the next day. At most of the counter serve restaurants desserts were pretty yucky anyway. And, try explaining THAT to a 5 year old.

Choose water to drink more often than soda. Many days we all didn't feel like eating a CS and a TS meal. We had a light breakfast in the room, we'd share a snack or two before lunch, many times we didn't feel like eating lunch but we'd force ourselves to in order to feel like we were getting our bang-for-the-buck on the "free" dining plan, then we'd do a TS dinner. Way too much food. We tried a buffet for breakfast one morning - we will not do one again. We stuffed ourselves almost to the point of being uncomfortable. I mean, who can resist Pooh's French toast!?
Here's what we'll probably do the next time we go:
Have breakfast in the room. Bring snacks. They should be high protein. Think CLIF bars, trail mix, PB sandwiches, apples and PB, bananas (put them in a stiff-sided ziploc type container to protect them), health-conscious granola bars, carrots and celery, etc. Take water bottles and fill them at the CS restaurants that give out free water (that water is filtered, the water from the fountains is not), take drink mix-ins like crystal light if you have picky children, eat a CS lunch at the park - sharing if possible. Buy a snack to share in the park. Do a late-ish dinner outside of the parks (utilize sites like groupon and restaurant.com to score some deals). If you are going to be eating TS meals in the park, lunch is usually cheaper than dinner. I don't believe CS meals change prices or menus for lunch vs. dinner.
I know you said you were going to leave during the day to rest. You know your family best, but for us it wasn't worth it. Of course, when we went the crowds were moderate, the weather wasn't too hot, and MK and DHS were closing somewhat early. Plus, our DS5 is not a night person. If you're going with heavy crowds, high heat, and longer park hours a break would be necessary.
Have fun!