Many considerations may be made in helping your student select an instrument. For example, the length of child's arms might affect the reach needed to move a trombone slide in and out. A student with braces might find a trumpet mouthpiece that applies pressure to be uncomfortable. A smaller student with limited lung capacity might have difficulty pushing air through the tuba.
Along with the student's abilities, your child's instructor will also work to balance the instrumentation of the group. A complete classroom of drummers and saxophones will limit the success of the entire group and your child's instructor may require extra expectations for specific instruments to help balance this.
Open communication in this instrument selection process is important. If your student feels strongly about a specific instrument, it is always helpful for their instructor to know this. Their excitement will increase their opportunity for success. However, your child’s instructor is the expert and has training and experience in considering all of the variables that might affect this success. Your child's success is dependent on balancing the instrument selection of their preferences with the instructor's advice for aptitude and classroom instrumentation.