From a musculoskeletal standpoint, the only real consideration would be to avoid excessively stretching the fingers and hands to reach the keys. With a child-sized instrument (?quint bassoon?) this would not be a problem, but the average 8-year old would not have hand breadth and finger length to handle a bassoon with compacted keys like the Fox 51 or the small Moosmann.
In reviewing the medical literature for other possible problems, studies are conflicting, especially in the dental and facial areas. Children 11-13 developed significant anterior tooth movements with wind instruments, and others showed wider dental arches. Other studies found little or no effect on tooth position or bone height in the upper and lower jaws. I could find nothing on potential musculoskeletal or respiratory problems from playing.
Unless an elementary school age child is playing bassoon many hours a day (unlikely even in the US), I doubt that these findings would be significant.
Dr. Bill Dawson, bassoonist and teacher
IDRS medical consultant
Past President, Performing Arts Medicine Assoc.
Author of "Fit as a Fiddle: The Musician's Guide to Playing Healthy"