For goodness sake, enlarged blood vessels in the penis do not signify that a man is either a pervert, a rapist, or a paedophile. The OP didn't even say it was a childrens' swimming lesson.
This may come as a shock to some of you, but the man on the bus, the man in the queue in the supermarket, the teacher in front of your kids' class, your dentist, the newsreader on TV, the RAC man, your own DH/P when at his desk at work may all have an erection at any given time.
And the famed 'morning glory' is frequently because a first thing in the morning piss is required!
And if he does find himself physically affected by being in a pool with skimpily clad women, so what? Multiple posts and threads on MN demonstrate that women often ogle at men and have innocent little sexual fantasies about the men they come across in their daily lives - how are men 'perverts' deserving of being complained of to the pool attendant, with consequent humilliation, in comparison?
you are the parent of boys they will have teenage moments when their erection does indeed appear to function independently - how would you feel if your poor boy was humiliated and cast as a sexual menace and pervert - peadophile, even, in a public pool?
Get a grip and act like grown up women.