Flex Adult lets you go in with up to 2 children under 16 any time for a whole year. You can attach 2 adults to the 2 children, but only one adult can go in with the 2 children at any time.
Previous Zoo Friends membership restricted entry to specified adult with up to 2 children.
It looks like Zoo Friends membership ($99) will be phased out when your current membership period expires (it's been extended a number of times due to lockdown) and replaced with Flex Adult ($110).
You need to pay $20 joining fee first time.
Great value if you can go to the zoo multiple times a year with the kids. Now if one adult needs a rest after lunch time, the other one can come in with the kids.