An appreciation of good music will help develop and maintain your child's interest. You don't need to spend thousands of dollars investing in a huge CD collection. By just having the radio station tuned in to the sounds of the great classical and jazz composers is an excellent way to develop knowledge and appreciation of good music. With rhythmic patterns, harmonics, and melodic ideas already well established in the child's musical ear, the transition into lessons will be an easier process because the child can see a well-defined goal for the lessons. Many parents get very frustrated because they don't have realistic expectations.
Children can only sit still for so long.
They have limited concentration skills as well. Problems arise when parents expect their children to behave as miniature adults. The child simply cannot meet these expectations and lesson time and practice time becomes pure torture. Don't let your expectations and desires be the sole determinant of when the child begins lessons or how fast you feel they should progress. There are many musical concepts that can be taught that actually involve physical movement.