I have four subscriptions and three are used by my children. I would like to have the ability to purchase a castle and have them all be able to use it.
My children range from 7 to 13 and they are not disiplined enough to save their gold so it would be great to be able to help them just like I do in real life.
Another nice family feature to add onto the parental controls is the ability to allow text chats between family members. This way my two <13 kids can learn to use it safely before being exposed to it when they turn 13.
As someone who pays for four subscriptions, it would be nice to have some additional features only available to family accounts, which would make paying for the subscriptions a little less painfull to the parent.