Firstly, I think it's a very healthy decision to take a break from the daily grind and reconnect with your family. Hats off to you for making it happen.

As for the car, personally I would store it. I don't feel that 3-6 months is too long to maintain rego, insurance and storage of a vehicle that you have some form of attachment to. Selling the car may leave you with residual feelings of regret or uncertainty with regards to returning to the city. If this is not the case, then by all means sell the car and allow the act to reinforce a fresh start for you and your family.