Are students having a good time and learning? Both are important.
Spend time with your child before and after lessons expecially before the child is comfortale with the instructor.
Watch but don't interfere with lessons. Then consistently apply what the instructor teaches.
At the outset, small classes (six kids to one instructor) may be preferable. They're fun, and kids learn interaction in a pool environment.
Kindness and patience are important for very young pupils. More experienced kids need an instructor with sound techniques, and a positive attitude.