I am a chef, who for 4 years cooked at a childcare center. I also raised 2 children. I know all about how kids eat. 
There are a few reasons why children are picky.  
First, they have more taste buds than adults. You loose taste buds as you age. Food tastes more intense to children than they do to us, that's why most children don't like black pepper, for example. 
Next, being picky about what they eat, is actually a self preservation mechanism for children. Way back maybe 1000's of years ago, being picky probably prevented children from eating something poisonous and dying. So, it's born in them. 
OK, what can you do? 
The first and most important thing is be a good example. Make sure you take the time to eat with your children. Even in our busy lives, it's important to sit down at the table and eat meals together. 
I don't think that happens as much as it should. 
Another point to know is it can take up to 20 times for a child to eat a "new" food. Be patient, but keep serving new items to your children, eventually they will eat it and most likely will like it. 
I suggest adding a new food to a meal with an item you know they like, say macaroni and cheese with broccoli or green beans, or a salad. If they see you eat it, they eventually will eat it too. 
Don't make a big deal about food. That's where a lot of problems can start, that don't have to happen. 
Children will eat when they are hungry. If they aren't going through a growing spurt, they may not have much of an appetite, but they would never starve themselves. 
Eating should be a pleasant experience, children as young as 1 can learn how to serve themselves food, which will entice them to try new things. If you want to know more about that, put in "Family Style Meal Service" in your search engine, there is a lot of information on it. 
There are a lot of recipes out there too that are "kid friendly" and nutritious. 
I hope this helps!