To resist the vending machines, keep something that is healthy and you like in your pocket or purse -- a protein bar, dried fruit, small bag of chopped up veggies, etc. If one is in the habit of keeping healthy food nearby, less chance to succumb to temptation.

To get into a healthier diet, replace things one at a time that are healthier but similar to what you are eliminating. Example: replace candy with dried cherries or prunes (not too many!)

Of course the best thing is start the children off with good habits at home. Don't reward good behavior with junk food or use it to pacify. Have the child involved in food prep and choices as early as possible.

I don't really know what I'm talking about because this isn't my specialty, I'm a tech guy with no kids, but still, you asked for thoughts and those are mine! :-)