Assuming you live in England there are various rules about children sitting in the front seat of a car at different ages and heights. A child may sit in the front seat as long they are over the age of three and use the correct car seat. Details of specific examples and exceptions can be found at: Childseats.org.uk.
However, summarily: A child of age three or below must be in a car seat (forward facing). When a child is between three and fourteen (or above a height of 1.35 meters) must use the correct restraint for their age and weight. Once a child is above 1.35m at 12, or after their fourteenth birthday they can sit anywhere in the passenger seats as long as they are wearing a seatbelt. In conclusion, a child may sit anywhere in the car, as long as they are using the proper restraints. If you are found to be breaking the rules and are stopped by the police you could be subject to a fine up to £500.
However, summarily: A child of age three or below must be in a car seat (forward facing). When a child is between three and fourteen (or above a height of 1.35 meters) must use the correct restraint for their age and weight. Once a child is above 1.35m at 12, or after their fourteenth birthday they can sit anywhere in the passenger seats as long as they are wearing a seatbelt. In conclusion, a child may sit anywhere in the car, as long as they are using the proper restraints. If you are found to be breaking the rules and are stopped by the police you could be subject to a fine up to £500.