Is the Elder Octopus Card the cheapest solution ?

I recommend each of you to buy an airport express pass plus an elderly octopus card. The airport express travel pass contains two rides on airport express (valid for 14 days so you can use it for back and forth to airport) and three connective days of unlimited ride on most metro lines. In this way, you can use the card to travel back and forth to airport and by metro freely for three days in HK. On the following days and to use ferry or bus, you can use the elderly octopus card.

BTW, the airport express travel pass with two airport express rides costs HKD 350 but you can get HKD50 back on refund. For elderly octopus, you need to pay HKD 50 as a refundable deposit, and HKD 20 as the minimum initial stored value; you may top it up if needed.