I think that this depends quite a lot on cirumstance. If you were in a public pool changing room on, say, a Saturday morning/afternoon, you would expect parents to bring their children in, and yes, that does mean sometimes girls.
I think in this case, being open (they will all become sexually active one day, so seeing a grown-man's penis should not be a big deal), yet descreet is the best course of action.
I personally, would not cover up, but might turn away when taking my underwear off/putting trunks on, etc. For some reason in my mind, seeing an arse is not as rude or provocative as seeing your cock and balls!
I think that there is no place for children in private gyms anyway, and perhaps after a certain time in the evening, say 8 o'clock, in public swimming baths.
I do however, enjoy being very 'open' with my nudity in changing rooms, as I like to think that I might encourage others to be more comfortable with themselves.
When you get a guy of say, 17, who is a keen swimmer, but wont show any flesh in the changing rooms, I think, 'lighten up a bit, and be relaxed with your body!'