A.P.
I have two sons, one is five and one is 7. They are beginning their 4th year of gymnastics and have not had any injuries to speak of. However, I have been called to school several times for injuries suffered on the playground and gym class. My point here, I guess, is that you can't predict when or if your child will be injured. It could even happen at a birthday party or a play date. The thing that we've learned with gymnastics is that the instructors anticipate injuries and know how to handle them. I would just make sure that they had your cell phone number and that you were accessible at all times. Two hours is agonizing for those younger siblings. Some of the mom's in my son's gumnastics class will actuall get sitters for their younger children periodically as well.
My youngest child has a medical condition that requires that I stay with him almost everywhere simply for the comfort of instructors, coaches, etc. We have had to come to grips with the fact that leaving him in school alone with a teacher is something that just has to happen. It's not easy for a parent to let go of a child so young and put them in the hands of someone else. We say a prayer at drop off and deal with whatever happens!