How much are the bike rentals and is there a discount per family amount ?.

It's like $32-$100 for a whole day, they have a huge range of rides. But the 32/40 $ ones are just fine for daytripping and touring the city. Not sure about a family discount, but they hook up a child seat on the bike for free if you need one. The staff is so super friendly though, just ask them about discounts. If there is one of any type, they will definitely hook you up. Check this out for more info: https://rentals.sportsbasement.com

I dont remember the price, but you have to count with an additional charge if you want to retur the bikes in Sausalito, it is not very clear when we rent the bikes

Is practically for free...