It's dependent on the size of the child seats as much as it is the car - especially if the seats will change as kids grow. Some cars will take two across a back row with an adult, but they're usually very large cars to accommodate the width of the child seats. This isn't exactly what you're looking for, but should be helpful in finding out which models offer the best seating space across the back row: heycar.co.uk/guides/best-cars-for-three-child-seats