At 6 and 4 they probably still have that tendency towards "I eat 5 things, and that's all" so you are doing pretty well. What I used to do was if they liked a healthy thing, like green pepper or chicken legs, I always had a LOT of that around, and kept running out of the stuff that wasn't good for them.
They will eat what's there when it's something they like.
Also, just a sidenote: I was cruising around the web and found that several people have discovered that their children have serious reactions to grains, yet their children seem to crave them; will want to only eat bread, waffles, etc.
What happened was the children were as addicted to the grains as any one is to sugar. Once they cut them completely out of their child's diet, the behavioral and digestive symptoms went away, and so did the child's cravings for grains.
I found this interesting because I've seen how children get into a rut, just elbow macaroni with butter, for instance, and that's all they want to eat. It's a craving, and addiction, since this is not that nourishing and the body thinks more will solve the problem.
I didn't raise my kids low carb, but I found that the less I allowed soda or Twinkies, the less they seemed to want it. Perhaps this will help with the waffles & bread issues.