5 people in a 172?!?!?
172 or 182, they can only take 4 pax (excluding the lap child under 2 thing).
Five people in a 172 is NOT legal nor is it safe! That could be considered a violation of § 91.13 "Careless or reckless operation (a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another."
It would certainly be a violation of "§ 91.107 Use of safety belts, shoulder harnesses, and child restraint systems. Each person on board a U.S.-registered civil aircraft must occupy an approved seat or berth with a safety belt and, if installed, shoulder harness, properly secured about him or her during movement on the surface, takeoff, and landing."
Remeber, your license is on the as PIC so don't try any stupid pilot tricks. With 4 people 172s are very near being over MTOW anyway. Two adults, 3 kids and full fuel with any bags would certainly push that envelope. Not to mention with 3 in back you are going to have a CG problem! Legality aside, it wouldn't be smart even if it were legal. I wouldn't worry about that 'child jumpseat' issue. If the plane had one installed, you'd have known about it already. Seems you don't have one, so it's not an option.
Break out your FARs or go here and check them out online-
- anyone sitting on the floor of an aircraft (like a 182) and using that as their seat must still wear a seat belt while not maneuvering for the jump as applicable in 91.107.