Steph - You don't say how old your kids are (other than school age), but it's never too early to start teaching them what is healthy and what is not. My girls have known since they could understand - we eat healthy in this house, and we have dessert or treats in moderation. For example, after a healthy breakfast they might find a Hershey's kiss in their lunch box. When they come home from school they are more likely to have cheese and crackers for a snack than they are to have a fresh baked cookie, but that does happen now and then. After supper, if they've eaten all their food without incident, we might have a scoop of ice cream, or this week a piece of Valentine candy - but we might say no.
We've always said, "We HAVE to eat at McDonalds" if we are on a car trip or in a hurry out running errands rather than , "Hey Kids!! Let's go to McDonalds - it's a special treat." Because we don't want them thinking that junk food is something to be coveted, but rather something to be endured. My daughter once asked me at 4 years old, "Mom, are these fries healthy?" I said, "Well, they're not really healthy, but we have healthy bodies so it won't hurt us to eat them once in a while." She replied, "I'm going to eat my chicken first, and then maybe I'll have more fries." Not that the chicken is that great for her either, but on her own she had figured out that it's best to eat what's healthy and leave the unhealthy for last. I was happy to see that she was grasping what I was trying to model for her.
I limit my kid's junk food and I tell them precisely why. When we make lunches we reference the hand-drawn food pyramid that's on the fridge and they each make sure they have one item from each level. (The new pyramid that has the fruits/veggies on the bottom and builds up from there.) They now at 6 and 8 can even tell me that they would rather eat at Subway than some burger joint because they don't feel well after eating a burger and fries! (Don't get me wrong, my 3 year old still begs for chicken nuggets whenever we drive past the golden arches!! But she's learning, too.)
You are the parent in charge of their good health. Teach them what is good and bad about foods so that they can make their own good choices as they grow up.