My children are now 11 and 7 and have very different eating habits.
When they were babies it was easy to get them to eat healthily as they had good appetites and would eat whatever I gave them, but when they were toddlers they started to "go off" fruit and vegetables so I had to be more sneaky!
One of the tricks was mashing broccoli and carrots into their sweet potato (my 7 year old daughter still has this now on her roast dinners).
My son stayed fussy for a while until he started to have school dinners where he tried food he would never try for me at home.
As he got older he became very sporty and aware that he has to eat healthily to be competitive. Yes he loves fast food and pizzas but he knows these are a treat and most of his meals consist of lean meat, veg and rice or a jacket potato.
My daughter is a bit more difficult to feed nutritiously as she is a very fussy eater. She would live on chicken nuggets, hot dog sausages, pasta and rice.
She is becoming sporty too and aware that she should be eating healthy so she will eat a plain meal of chicken breast, rice and veg (although her dramatic performance when eating the broccoli is worthy of a Oscar!).
Letting her cook with me helps too, if she has chopped the carrots she is more likely to eat them.
Another thing she has started doing (her idea) is writing a menu for her dinner. She picks one protein item, one carb and 3 vegetables.
They have a sweet treat after dinner which can be a yoghurt or small chocolate bar like a Kinder. They also have a Macdonalds or Burger King, 2 or three times a month and I let them have sweets etc at parties or on days out as denying them completely could effect their relationship with food.
Even though they are aware of what is good/bad for you, they do ask for treats such as crisps and chocolate, especially when we are sat watching TV together in the evenings. They declare "I'm hungry ", but when I suggest they have some fruit they make excuses. (I get it to be honest, I often need that Kit Kat instead of an apple so why wouldn't they!)
I have therefore started making sugar free jelly with raspberries, mandarins or grapes for these occasions or if we are out of jelly, they will have a handful of sultanas.
We talk openly about food and nutrition and the children are aware of what each food group does for your body. I have always put the focus on what healthy/unhealthy foods do for the inside of our bodies rather than making an issue of the outside signs of unhealthy eating such an weight problems.