Pay close attention to what they will eat.

The finicky eater is every parent’s bete noir. With a bit of persitance we can turn our child/adversary into a model of health.
Even the most picky child will have a small group of foods he or she enjoys. Make a list of these foods. To this list add those foods the child will eat, even though he or she may not enjoy them. Look through this list for foods that form a balanced diet.
You now have enough information to build a diet. To this diet add very small portions of new foods at least once a day.
Over time the list will grow. Over the course of a year your child will have a diet that, if boring, is healthy.