Or, consider learning to play the instrument with your child. Praise your child for each step forward. Never make negative remarks about how your child's playing sounds. It takes time and effort to produce musical sounds. Encourage other family members to applaud the child's efforts.
Positive attention is a great motivator.
Remember that there are always peaks and valleys in the learning process. You and your child should expect times of discouragement, accept them, and focus on the positive fact that she's learning to make music. Remind her that everything worth doing takes time and effort. Provide positive role models. Bring your child to hear amateur or professional musicians perform.