The first link is on the US site, and states (in the fine print) "To book a family room, or a second room at half price for your children, call the dedicated booking line at 00800 44 45 86 67." The phone number looks like a UK number.

The second link is on the Doubletree UK site, and states "kids stay for free when sharing your room, or can have a second room for half price" and "* to make a family booking please call our reservation centre on 0870 896 0606." Again, this is a UK phone number.

I noted in this thread that "Hilton Barbados offered the second room for half-price for my kids a couple of years ago. I found the info on the Hilton UK site, called the Diamond Desk, and they were able to make the reservation for me. They did mention it needed an "unusual" booking code. Definitely cannot book this online." The offer is not limited to the UK, but I don't know how far it extends.