I find the best ways of helping young children gain confidence are to encourage and nurture their interests and skills. If they're good at drawing and painting, for example, admire certain aspects of their artwork, rather than the whole picture. This way they know that your compliment is genuine, rather than exaggerated.
It also helps If you can encourage them to solve their own problems rather than rushing in to help. For example, if another child has taken some of their Lego blocks. Rather than demanding the other child gives the blocks back, ask them how they think they can resolve the matter, and discuss different ways of dealing with the issue.
These are great ways of helping children to gain self esteem, and building confidence for the future.