My mom was a stickler for manners, and on the top ten list was this one - "It's very rude to point out to someone else they they are being rude".
I f-ing hate this rule!
But, heres a story for you: Back in the day when a gentlemen would never take off his dinner jacket before his host did, a man showed up a a party with no jacket. One of the guests commented to the host - "my, what an uneducated chap!". To which the host responded "well, it is rather hot today", and took off his own jacket.
So, while breaking the jacket rule was a mistake, an even bigger mistake was pointing out someone else's mistake.
Anyhow, as far as I know dropping hints is okay, so long as you keep your voice polite. Seeing the kids abuse the starfish, I might be tempted to warn the mothers of all the horrible bacteria and parasites on the animals. For the children's own good, they should leave the starfish in the pool.