I don't want to sound insulting so please forgive me if this doesn't come off right.....
Your kids sound normal. They sound like people who will not have weight issues in the future because they have the ability to NOT eat when they're NOT hungry. Don't fight them on this. Any doctor will tell you a kid will not starve themselves. (and if you're concerned this is a metabolic issue,anorexia, or another physical health problem then consult a physician).
I want to say this very gently: don't force food on them. Don't make it an issue. Offer healthy foods at reasonable times and then let them control what goes into their bodies.
I am assume for the most part the people here on this site are 'broken' in regards to their ability to see eating and food in a 'normal' way. I certainly am broken. I look at my family and I am mystified at how they can leave a tablespoon of milk in the carton. Just drink it, I want to shout! Well, that's not normal for them. They think simply in terms of what they need to satisfy their hunger and only take that amount. I boggles me that my husband and I can have the same lunch and he won't be hungry for dinner later that night while I'm counting the hours till I can eat again.
Anyway, again I apologize if I'm off the mark. Just my point of view....