I thought it might be a worthwhile resource - I know it has made a big difference to me on a few occasions we've needed it so far...
1) Radisson SAS. Second room at half price and will let you pick two different rates, so room only for the kids if that helps
2) Hyatt. Have just snagged this at Orlando Airport. Is half the best prevailing rate so some pre-paid deals may work out better.
3) Hilton. At least in the UK second room is half price, but unlike the Radisson deal you have to pick the same deal on both rooms.
4) Some Marriott properties, specifically those in Florida, seem to offer this under the FML promo code. Also Ritz Carlton in the US & Carib.
It also seems that the following don't do it:
1) Holiday Inn
3) Marriott in the UK.
Any additions or comments would be welcome.