The biggest part of cooking for the kids is going to probably getting them to understand that you are aiming for a healthier lifestyle in which to live longer for them as well as help them avoid having to do the same things in the future. Once that's drilled in for them to understand and you can show them some examples of how they could end up (overexaggerate it and find photos and videos on the internet that will make them want to change immediately) then they will be more than supportive of your family's transition.
While doing so, you want to have some "fun days" for them to maybe choose what kind of food they want to eat (for example pizza... tacos... etc.) but cook it while they are not in the kitchen with you. That'll be your chance to swap out ingredients and build something much healthier and they won't even know it so they can't not want to try it. After they gobble it down (hopefully) then you can say, hey... that wasn't what you thought it was!!! I see you all enjoyed my **** mixed with **** eh!? They'll be surprised and might even actually advocate the newer food to where they prefer it and adopt it as part of their own future regimens.