Explaining To A Child Why He Needs To Eat Healthy Food And Not Junk Food?

First they are the child and parents need to be parents.  Sometimes it is a matter of saying no and substituting a healthy food into the place of the junk food.  When your shopping go to the cereal section and let your child point out some of the television advertised bands.  When you child picks a healthy Corn Flake because they recognize the picture, tell them how smart they are to pick a food that is good for them.  Hesitate when the bad food is wanted.  Let them shop for gummy fruit bears and they will start addressing they like to shop for these foods because your going to let them have and shop for them.
Don't eat a candy bar and tell your child not too.  Purchase healthy finger foods and keep them on hand and if your child is having a fast food break down get them a healthy snack.  You can't just sit down and tell a child to stop, everything they like is bad and they need to go on a diet.  They will not understand the concept.  You must talk to them with the actions you take when they are wanting a McDonald's toy so lets go to McDonalds right now!  Explain to them you know they want that fast food just because it has a toy.  Go into the toy box get a toy and sit down with them with apple slices and feed them, the toy and yourself.  They would much rather eat and play with you than eat junk foo.  Spend some time reading food books.  You can get them through your local WIC office, that teaches good foods to eat.  I use a magizine when I have over weight children in counseling.  We open the book and start trying to find good food to eat in the pictures.  If one of us plays like we grabbing fast food the other one gets to scold and they began to relate good/bad.