OP, I think talking about what you want to do & experiencing what will really happen are two totally different realities. I could be wrong, of course, & definitely won't be the first time.
However, even though you have children who only like McDs, don't you think there is a big possibility that they will want a taste of something they see at CM? I find it hard to believe that they will not spy
something they want - chocolate desserts maybe? Or other desserts - choc chip cookies? Bread? Mac & cheese? I know there is a child's buffet & maybe there are hamburgs & fries on there - aka McD! I just wouldn't feel comfortable or enjoy my meal while telling my child, "No, you can only watch me eat!" Oh, that sounds so wrong to me, & a big possibility of what could happen. Do you really want to do that? I realize you will explain the situation to them beforehand (when they choose McD), but they are children, not adults, so is it reasonable to expect a child to envision the future situation at CM.
I also understand you not wanting to pay full child price for children who will only pick at a few items.
So, to me, you have the choice of
1. going earlier & paying for them (every time we've gone, walkins were always accepted if you get there when they open), or
2. giving them an apple to hold them off until their buffet meal (& you pay the money you don't want to pay), or
3. sending them to WDW child care (costs money), or
4. you & your husband each go alone while the other stays with the children, or
5. not going at all. I have no doubt that you are familiar with these type of selfless decisions we parents must make - as painful as such decisions can be sometimes, especially if you love crab legs as well as my DH loves crab legs!
I think if saving the child care money is important, I'd go for #4. After wearing yourself out at the parks & tending to your family all day, a nice quiet meal alone with only yourself to think about may be better than a spa treatment!

Ahhh, envision, slowly & quietly raising that well buttered crab leg to your mouth without anyone saying, "Mommy, I have to go to the bathroom!".
Enjoy your trip.