One of the most common ways that parents try to encourage their child to eat something healthy is to hold the pudding hostage... "If you don't eat your salad you can't have any pudding". Superficially, this makes sense. Why would you let a child fill up on ice cream when she had rejected her healthy main course? Research shows that making the dessert conditional like this increases a child's vegetable consumption in the short term. And this is the critical bit: in the short term. At that meal, she eats her salad, gets her ice cream, job done. However - in the long term, scientists have found that making one food conditional on another increases a child's dislike of the 'hard work' food and increases their liking of the reward food. Back to the table: yes, she has eaten her salad which she may well not have done otherwise, but at a cost. By the end of the meal, the idea that salad is hard work has been reinforced and your child dislikes it even more.
When you are dealing with a picky eater, anxiety about your child's food intake is understandable and natural. First, check his or her growth and weight with a medical professional. This is a really important starting point because once you know that his growth and weight are normal, you can begin to be a little more laid back about what's going into his mouth. Whatever your child chooses to leave, stay relaxed. Bargaining with the dessert will make picky eating worse.