When you search or book, you say who is travelling: adults (16 and over), children (5 to 15) and under-5s. Under-5s always travel free and are never charged.
Some operators set a child price for their listing. Where they have, you will see a separate stepper for children and a lower price for each child. Where they have not, a child takes the same seat rate as an adult on per-person services such as tours and charters, and makes no difference to the price of a room or a vehicle.
For rooms, you can add more than one room type to a booking. Each room card shows how many are left when there are three or fewer.