Upvote:3
You don't say the location from where you are making your applications, but you are able to book a group appointment at a visa application centre. It is a fee-based service and the additional costs quoted to you may be in line with the fees associated either with a group appointment, or with not booking in advance and the centre accommodating a walk-in group.
Those travelling together, including families, may attend group appointments but each person applies and is assessed individually for a visa. A minor may be accompanied by a parent, of course, but it is incorrect to assume that all family/group members may simply appear for an appointment booked for one individual.