The 9yo child must use the correct child car seat (which, in practice, means a booster seat or cushion, conforming to the regulations, in conjunction with an adult seatbelt). One of the other children must use the second seatbelt. The third child can (legally) remain unrestrained but it would still be highly inadvisable.
http://www.childcarseats.org.uk/the-law/cars-taxis-private-hire-vehicles-vans-and-goods-vehicles/
If it's only a few miles (and therefore, presumably, a fairly quick journey) it would make more sense to take the adult (whom I assume will be occupying the front passenger seat) and the 9yo (using a proper booster cushion) in one journey and to take the 12yo & 13yo in a separate journey. (The order that you do those journeys being determined by which end of the journey is the best place to leave the 12yo & 13yo alone for a short while).