Booking for a family is a different problem from booking for yourself. Two details cause most of the trouble.
Seats are not the same as people
A car listed as six-seater usually counts the driver. Book on the number of passengers travelling, and if the last row is a fold-down bench, treat it as short-journey seating rather than a comfortable seat for several hours.
Luggage is the real constraint
Space runs out before seats do. A full boot plus a full back seat is the usual failure, and it is discovered at the kerb with everyone already standing outside. Count the large bags and book to that.
Children and car seats
If you need a child seat, arrange it when booking rather than on the day. Say the child's age, as the requirement is not the same for a toddler and a six-year-old.
Long journeys need breaks
On anything over a few hours, plan stops. Travelling with children, plan more of them than you think you need.
