Totally get what you're saying... it's hard isn't it?!
I have stopped my kids from mainly eating carbs (toast) in the mornings and asked them to keep the bread to a lunchtime only, which they agree to. When they get home they have boiled eggs with a little mayo. I'm working on making their lunches healthier, though two pieces of bread a day, compared to cereal, toast, then sandwiches, then more toast after school (or whatever they can inhale from the fridge!) and then potatoes etc at night is a win no matter how you look at it.
If you have control over the morning and evening meal, you're essentially two thirds of the way there.
I'm interested to know the answer to this too. Suspect the answer is that if you increase fats along with carbs, then the eater will put on weight ...