I just wrote a post on this under the Park Hyatt Vendome hotel review.

Hyatt does have a policy documented, but its not intuitive on where to find it. If you are on their website, and go to select number of guests in the room. You will see the options for Adult & Child. You have to click on the "i" next to Child to then get the pop up window and read the fine print.

In the US, Hyatt determines a child as anyone under 18. In the rest of the world a child is defined as 12 and under. If you have a child great than 12 years old but less than 18 and you add them as an adult, your reservation will then be for 3 adults.

You will then see the price options listed for your hotel. You can then set the options to be 2 adults + 1 child and get the price listing. Compare the two room prices to determine how much the hotel is charging for that "extra adult" in the room.

The Park Hyatt Vendome in Paris adds on 100 Euros extra per night. The Hyatt Churchill in London adds on 30 GBP's extra per night. It just varies by hotel.

However, if you want to book a P+C rate, you have to call the Hyatt reservation line to book and if you just say 2 Adults + 1 Child, the agent will most likely not ask your child's age and even if they do they will most likely not know that Hyatt hotels outside the US will charge extra.

I have raised this with Hyatt as this could lead to some big suprises for guests when they check in and find out they have to pay $100's of dollars more than they were expecting.