I'm not sure that this is entirely true. It depends how many kids you have in the car. I copied this from
citizensinformation.ie . .
The requirement in relation to rear seats in cars takes account of larger families in the following way - where 3 or more children under the age of 15 are travelling in a car, the requirement to wear a safety belt or use an appropriate child restraint is complied with, in respect of the rear seat, if every reasonable effort is made to buckle in as many of the rear seat passengers as possible. No doubling up on a front seat is permissible but if, for example, 4 children occupy a rear bench seat designed for 3 adults, it would not be possible for all 4 to be buckled in unless the vehicle has been modified. While it is not unreasonable for a vehicle owner to make such a modification, it might, perhaps, be considered unreasonable for the law to expect that such a modification would have been carried out in every case where a family of 2 adults and 4 children use a standard family car.