Most hotels at Gatwick have designated family rooms which will accommodate 2 adults and 2 children under the age of sixteen. Some will have a double bed and 2 single beds but often a double sofa bed will be made up for the children to share. Alternatively, some hotels offer interconnecting family rooms with a double and twin room linked by an internal door.
At most hotels a family may book a double room with an extra roll away bed if there is only one child and that child is aged 12 years or under. There may be an additional supplement for the bed but generally it is more economical than booking a family room which will accommodate 2 adults and 2 children.
If your hotel search includes one or two adults and a child aged 2 or under, the results will automatically allocate a cot for that child. You can also place a request for a cot in the booking summary if you would prefer your child to sleep in a cot rather than a bed.