I guess I look at it like this. Disney wants to be nice and they will let people have a lot of leeway, so can you get the kids in there without paying, probably, should you? Probably not. It's not just the food you pay for there, but the ambiance right? I could not see going to any sit down restaurant, at home or on vacation, and not order anything for my kids. You said they would prefer a quick service meal, I assume this is because they don't really care to have a longer meal, or the food at the restaurant is something they will not eat, at this point you just say, tough this is what you're eating and you will be patient OR you take your family there. Taking up 4 seats at a restaurant that is very busy and always full and hoping not to pay for 2 of them is not really fair to the other patrons, on top of that, it sounds like you will have 2 impatient kids with you, that doesn't sound like a very good dining experience for you, the server(who is in an awkward position of now having to police if you slip any food to the kids, again something they don't want to deal with because they are in a lose/lose situation), and the other patrons around you should the kids start to get vocal.
That is not to say I don't feel for you, I have kids too and there are times we just cancelled a reservation because we knew the kids would not like it and I had really wanted to go there, heck this is a constant battle in my home lol. The good thing is Ohana isn't going anywhere and will be there when the kids are ready for it.