In a nutshell:
Add. Don't subtract. Crowd out junk food by offering as much fruit and vegetables as possible.
This is a good season for getting this particular ball rolling, as the full spectrum of deliciousness is coming to harvest and will be around for a while. Load up!
Mom tip: ask your kids to help you put together a rainbow on a plate. Most kids will jump at the chance and then they will usually be so invested in the process that they will want to eat it all too. Or, at least try it all. It's worth a shot.