For some children, it might be easiest to start with a food that has a similar flavor (trying fresh strawberries if a child likes strawberry ice cream); for others the texture may be the most important issue (adding corn chips if a child likes potato chips).
If you do have concerns about nutrition, it's fine to give your child multivitamins. Choose the type your child is most likely to tolerate, be they crunchy, chewy or liquid.

