Off the top of my head (it's a mental list and some saved pages/bookmarks I'd have to track down):
Scratch (MIT)
Alice (Cornell, IIRC -- just saw that an update is due next week)
One or more versions of Logo (e.g Berkeley) (recently, there's at least one that's hosted in the browser; might lower the gap since no explicit installation is required).
A couple of things I'm forgetting right now.
When I run across these or particularly a nice introduction to one or another, I forward the page on to the parents FYI. When/if they start to respond with "tell us more", I'll go more in depth (and excavate those pages and bookmarks).
Suggestions welcome. Since they're not my kids, my approach has been somewhat lackadaisical.
