At some point, when you are figuring out increasingly unlikely accidents, maybe you should stop.
A more likely danger is that a non swimmer is drowning and grabs onto another child and drags them underwater or tries to climb on top of them. This is a known danger and is a standard part of lifeguard training in the US. Drowning persons can be very dangerous to other people and to rescuers. So a drowning child can definitely endanger other kids.
Cmon, we're not considering nuclear war and drowning in the radiation soaked pool ?
Based on https://what-if.xkcd.com/29/ I tend to think that a pool would be safer than out of the water in the event of radiation being a problem:)
I was a lifeguard for a decade bud, I’ve seen it all.