The thing to remember is all foods are in moderation.

If you ate too many carrots it would make you sick. If you ate too much celery it wouldn't be good for you (the sodium alone is bad!). Same for meat and everything else.

We aren't supposed to eat the same thing day in day out over and over again. Variety is really important. And that means sometimes there will be meals that have higher fat content than others, and sometimes you will not eat much at all. It's better to teach your children to listen to their metabolism, focus on eating well rounded meals and drinking lots of water than it is to focus on bad foods. I've seen what that can do. Out of all the children that my kids are friends with, it's the ones that aren't allowed anything that go nuts and make themselves sick at the party table. Out of the kids I knew in high school it's the ones who's parents were strict that now have food related health issues. Yes you don't live to eat, but I think by making food a journey and allowing them to explore new things with their taste buds, you'll find that "junk food" will be an absolute turn off because there is so much more available to them. I know I'd prefer a 6 course degustation over McDonalds any day.