That depends on the policy of a particular hotel. If you request 2 adults + 1 child (2+ y.o. needs a separate bed), you should choose the rate plan "double with extra bed" or "triple". If there is no additional extra bed in the rate, you will see this remark displayed.
If the system shows you available Double rooms without an extra bed, it's more likely the hotel will request an additional charge for the extra bed. In such cases you can contact our support team by email firstname.lastname@example.org to clarify the details.
During the booking process, you can request an extra bed/baby cot by leaving a note in the “Your requests” field: