What card we will use?

my family and i will be going to HK for 4days. destinations are(kowloon,tsim sha tsui,lantau island,mongkok,yau ma tae,mid level). We have 2 kids(5yr old and 8months old)? are we required to buy on loan octopus for each of them? what is the suggested kind of card that adults are required to use?

Thanks