The oil does not break down and can remain toxic to us for long periods. So if you don't get it off the dogs it'll stay there until something causes the oil to be removed. Dogs being what they are will probably remove at least some of the oil through their normal daily routine of rolling, running, bumping, etc. A good rinse should dilute and remove most of the oil. We've had pretty good success with our dog with rinsing him down with the outside hose when alternatives are not available.

As for the car seat question - well that depends. Urushiol oil will bond with the skin and once bonded there is no way to spread it (ie contaminate the car seat). BUT if there is oil on your skin that has not bonded it can indeed be transferred to other items. Just to err on the side of caution, I would clean up the car seat as much as possible.