Happens all the time at my work. There is no lifeguard on duty because its majority swim school with a small public swim section so parents are entirely responsible for their childs safety when they are swimming, ESP in the 25m pool. The amount of times we instructors have had to rescue kids way out of their depth because their parents arent watching them is ridiculous.

If your child cannot swim the length of a pool (25mtre) without a floatation device then they are DEFINATELY not ready to be unsupervised even for a minute; Nai brought up a good point about being tall enough to stand with their nose/mouth out of the water.

Thankgod you were there Nai!!
It surprises me that your work can do that. I'd have though the parents supervision thing would my stand up in court - if the pool aren't providing supervision then they should not let kids in who are not in a lesson.