The 50% off kids room is a benefit I love, and use a lot.

This year I have used it at the Conrad Tokyo, Conrad Hong Kong, Hilton Wangfujing, Hilton Antwerp, Rome Cavalieri and at the London Park Lane (checking in there in December though). But I have used it at many other hotels as well.

This is the link.

A few tips come to mind:

1. If you call the HH desk to reserve 2 rooms, both rooms will have the "normal" non discounted rate. The 50% off is usually mentioned in the comments area.
2. Do check your reservations, because the hotel will change the rate for 1 of the rooms.
3. If not, then contact the hotel and ask for the rate change using the link above, or, skip steps 1 and 2 and work with the hotel directly.
4. We usually reserve a suite and a connecting room; we pay the normal rate for the suite and get 50% off the room rate. That 50% should be based on the room rate, not on the suite rate.
5. Sometimes it is cheaper to get a prepaid rate, but we prefer flexible rates. The promo doesn't work with prepaid rates, but with flexible rates (with or without breakfast/dinner.)

Hope this helps.