Stay close to downtown, don't stray far, and make sure your kids are buddied up and have responsible chaperones with them.
Never for one minute did I feel unsafe in Atlanta, anywhere.
In St Louis that was simply not the case.
That said, obviously the Arch and the museum under it.
Our high school team went to the Royal Dumpe dinner theatre (
http://www.royaldumpe.com/) - they toned down the show for us, but it was still fantastic.
Riverboat tours are pretty cool, too.
Union station is also pretty neat if you need to fill a few hours.