Educate them at the early age. Share our knowledge about all type of food, tell them which one is healthy, which one is not. Personally, I will give them a book to read about all kind of food, with all the benefit of the food itself. Build their mind with the healthy lifestyle. Nowadays, a lot of children like to eat junk food, not because it is fast and tasty, but because of they are cheaper than fresh food. So, it is better to tell them the consequences of eating junk food, give them a video about the results of eating food that are not fresh.
Sometimes people do not know which one that can be recycled which one is not. Other than plastic and cans. I think the best way is to show them correctly, and explain them what is going to happen if they ignore this. In example in Jakarta, Indonesia there is 4,500 tons of rubbish (which is 4,081,500 Kg) every single day, it caused flood everywhere. So, you could imagine if one family change and throw the rubbish on the right place, it will bring the impact for their children and also their neighbour.