Don't worry about it right now. If there's truly a lack in their diet the pediatrician would give some sort of supplement to help them along. Right now, just be satisfied that their eating. It's only a phase. Something that works for me once in a blue moon though is to pretend that it's your serving of green beans or spinach. Kids tend to want what their parents have at that age. But like I said it only works once in a blue moon.
I ALWAYS GIVE HIM THE GREENS FIRST AT MEALTIME IF HE DOESNT LIKE IT AND THEN HE THINKS THAT IT IS THE ONLY THING HE GETS AND EATS IT. THEN LATER I GIVE HIM WHAT HE LIKES.