As implied by erik123, it depends on whether you sleep with your kid or not.

Lots of hotels don't charge for an extra child up to X years (value of x varies) as long as they use the same bedding as the adults. Whenever this was the case, we never added the child to the booking. With reward nights and offers it was often the case that rooms for 2 adults and a child weren't available (or were hugely more expensive) but the cost was more than reasonable when just two grown ups were on the reservation. Us bringing the unbooked child along - never needed or requested a cot - was never questioned.