Probably should have been working on a fake ID long before this, not one that will allow you to drink, but at least one that covers you for shows...
anyway, I haven't noticed kiddies being rounded up, though I have seen an uptick of mother/daughter pairings at least it seems like an uptick.
They are probably just covering their ass with that notice, seems really difficult to enforce unless they had a 17 and under section that they could herd out all at once, I wouldn't fret this yet, if it were happening we would have heard about it by now.
The clubs want you there for the cover, and cola sales, if nothing else, so I can't see them in a hurry to start some shit about it, but they are right in posting that if it's the law...