Make a list of what they *will* eat, and not the premade, prewrapped stuff.
Then, make sure one of the things on your list of 'will eats' is on their plate. Then, serve the main dish that you've prepared. I usually have three things on my son's plate, and one if for sure something he's eaten happily before. "That's dinner."
I don't talk about it. Don't cajole or coax. What's on the plate is what's for dinner. I don't cut crusts off-- but you don't have to eat them either. "Eat up to the crust." is fine. When I served snack at my preschool and a child complained "I don't like that"... well, 'this is what we have for snack'. Said in a calm, very 'matter of fact' tone of voice.
My son loves mac-n-cheese, so I allow one box of it per week. He can eat it all at once (yikes!) or have a bit with meals. But ONE box per week. Allow the 'junk' food within moderation, and be clear about your limits. A small serving of pasta with olive oil and Parmesan is offered sometimes, too, but as I said, it's not cups and cups of it...there are other foods on the plate to eat too.
I might offend someone, but here goes: it's really hard to pack healthy lunches sometimes when sometimes other parents pack junk. It creates a food envy for little kids when they see their peers getting a cookie, chips, a piece of fruit or go-gurt and a sandwich and a juice box. Or worse, lunchables. Hell, those things don't look like food, they look like a present or an activity. I had to send home pretty clear directions with my preschool families that NO sweets were allowed: fruit was fine, but no yogurts, go-gurts, cookies, etc. It was a good choice for us and the children ate their lunches.
Keep serving reasonable meals, J.. I'd also suggest NOT packing a treat in the lunch. Very simply explain "You are not eating your lunch, you are filling up on treats, so I'm not packing them any more." Have one or two treats on the weekends, but no desserts during the week. Make sure they are getting things that look good, too: a ploughman's lunch of hardboiled egg, whole-grain bread + butter or some whole grain crackers, cheese and apple slices--throw in a carrot with some peanut butter....
Remember that adult 'healthy' and children's healthy are somewhat different, too. They need healthy fats (black olives are great and a favorite), more carbs than we might choose, and good protein sources. I never make my son take a bite of anything he thinks is 'yucky', but overall, we have a pretty good eater. Also consider if the kids are exposed to advertising which promises all the foods they are asking for. Ad executives are savvy students of child psychology and KNOW how to entice kids to beg and nag for the junk food that's being advertised to them. Cut out the treats (except on weekends), offer moderate and balanced choices, and think about what their influences are.... eventually, they should come around. If not, time for a talk with the pediatrician.