OK this is a few bits from the directive
In summary, the new Directive requires the following changes to be made to the seat belt wearing or construction and use regulations:
all children under 3 years old must use an appropriate child restraint when travelling in a car or goods vehicle and may not travel unrestrained unless an appropriate child restraint is unavailable in a taxi;
children aged 3 or more years old and up to 150 cms (approximately 4 feet 11 inches) in height must use a child restraint appropriate to their size when travelling in cars or goods vehicles fitted with seat belts. Some exceptions are permitted. In vehicles not fitted with safety systems they may not travel in the front;
rear-facing baby seats must not be used in a seat protected by an active frontal air-bag;
all child restraints in use must conform to UN-ECE Regulation 44/03 or Directive 77/541/EEC requirements, or any subsequently agreed standards;
and all occupants of cars and goods vehicles must use seat belts, where provided while seated and the vehicle is in motion, and the number of people carried in such vehicles may not exceed the number of seats available fitted with seat belts or child restraints
these are some exemptions - most importantly for when you have three or more children
The Directive gives Member States the option to reduce the 150 cms height threshold (above which children can use adult seat belts) to 135 cms (roughly 4ft 5 inches). It also provides an option to allow all children to travel without child restraints in the rear of taxis if restraints are not available.
The Department considers that the amending regulations should provide for the exemptions permitted under the Directive. These recognise some of the practical difficulties that arise from the requirement to use child restraints. Therefore we propose that from 9 May 2006, all children aged 3 and over and up to 135 cms in height when travelling in cars and goods vehicles must use an appropriate child restraint except in the following circumstances:
when travelling in a licensed taxi or private hire vehicle being used to carry passengers for hire and an appropriate child restraint is not available, providing they do not occupy a front seat;
when a third child is being carried and two child restraints are already fitted in the back of a car or light van and lack of space prevents the fitting of a third child restraint. In such circumstance the third child must use an adult seat belt in the rear of the vehicle;
and when no child restraint or an insufficient number are available, an adult seat belt may be used in the rear of cars and light vans on occasional journeys over a short distance.
You can read it in full here