Where to leave your car while in Sesimbra
In Sesimbra there are very few free on-street parking spots. The locals seem to take it as a question of honor to find one, even if they do have a garage (which they use for storage). For short stays the metered lots are very convenient and not that expensive. Do pay the Euro or two! The city contractor is out there and I have seen many a ticket!
The other options: At the entrance to town from Avenida da Liberdade you have two lots that are €2,- for the day (https://goo.gl/maps/m9nj1rSvHMU2). You usually can stop in front of your hotel or apartment to leave your luggage and the kids and drive back to the lot to leave your car there. In Sesimbra you will not need a car.
Sana Sesimbra and Hotel Spa have a parking garage that you can use as well - even if you are not staying there. It runs around €20,- for the day.
During the off season it is easier to find a spot on the street, but check for the signs, not all spots are legal and the municipality does tow - especially from handicapped spots!.