As usual the advice (particularly that given by the AA) is confused as it doesn't properly consider vehicles old enough to not be fitted with seatbelts. To quote from the original DirectGov reference above:
[h=3]"If your vehicle has no seat belts[/h] You can't carry any children under three years old in vehicles without seatbelts, like classic cars. If you're travelling with children over three years old, they must only sit in the back seats."
Andy