It's always a delicate balancing act. I have three girls (7.5, 5 and 4) who are, respectively, athletic with some pudge, tall and skinny and a little block of muscle.
Since I'm a recovering anorexic, my weight sometimes becomes a bigger issue than it needs to be. Our family is no stranger to mental illness, though, so my husband has honestly explained the illness to our children (in a way they can understand).
I'll try not to ramble while I explain how we approach it in all other venues.
Main Point: Honesty, honesty, honesty.
1) We stress good foods for health and fitness. You don't eat well to stay pretty and thin, you eat well so you have energy to sled, hike, play with the dog, toss around a frisbee, swim and so forth.
2) We explain that some people gain weight and some people don't, just like some people have blonde hair and others have brown, or some are short and some are tall. It's extremely important to us that our children know it
is possible to be slightly heavier while still eating good food, and, if that's the case, it's fate and they shouldn't worry about it.
3) My oldest inevitably brings up my weight loss efforts - and the low carbing. I've told her I'm trying to lose the weight I gained by eating bad foods. Since she's a pretty smart cookie, I've been successfully able to get her to understand (at a low level) the metabolic damage I've done to myself by overeating candies, pastries and other sweets. She understands that, because I overloaded my body with all this junk, my body reacts differently to a bowl of oatmeal than hers does. "I gained weight by eating foods that weren't good for my body, and those foods hurt my body. Now I have to fix it. Remember what Daddy said? If you eat good foods, it's not how much you weigh but how you feel. You only need to worry about how much you weigh when you're eating foods that aren't good for your body."
4) Out of all this, we reinforce the positive. Our kids know that they eat "good" foods the vast majority of the time to keep their bodies healthy and fit. They also understand that if they don't keep their bodies in tip-top shape, they won't be able to have the sweets or pastries as a treat.
How do we do this in the home? I mean, how do we
actually accomplish it?
We don't keep sweets in the house. If I have chocolate, it's extra-dark chocolate. I admit to having a "stash" of milk chocolate in the back of the pantry, which is given out as a treat after I introduce a new (and usually) bizarre food. Maybe food-as-a-reward is a bad thing to do, but I figure my kids deserve something they
know they'll like after I put a spicy lentil dish on their plate!
I don't buy cookies, cakes, ice creams or just about any other candy that can be overeaten. I tell them it's because I have a limited grocery budget and don't want to spend $5 on a package of Oreos. This is true, but I do skip the part about not wanting the junk in my house. If they want cookies, and have been good, I'll put in the effort to make a batch (with fresh-ground oatmeal flour rather than white flour, I should add

). Ice cream is a treat for a day out or an afternoon with grandma and grandpa, and cake is something you get on birthdays and holidays.
I don't buy prepackaged meals unless someone is sick. We ate a whole lot of frozen Bertoli meals after my son was born, but, for the most part, I cook everything from scratch or nearly scratch. My kids watch me cook and see what goes in from beginning to end. My oldest daughter has naturally taken a liking to reading the ingredients list. "What is dipho-hoda-watcha-max? Do we have any of
that in the spice drawer?"
If my kids ask for a fruit or veggie at the grocery store, I
always find room in my budget for it. Some weeks we go without bread (by "without" I mean store-bought, I bake bread as needed for lunches) or improvise the cut of meat I wanted just to make it possible. Unless the item is twice what I'm willing to pay, I put something back if my kids want it. This week is was Starfruit for my middle and pomegranates for my eldest. And, once again this week...we have no bread, lol.
My kids aren't allowed to gorge themselves on carbs. I don't force low-carb on them, but carbs do raise blood sugar and Type II diabetes has been popping up in my family. They get one serving of the carbs and have to fill up on meat or veggies if they're still hungry.