I can see the idea here, but accounts are treated on an individual basis with a part of the agreement saying accounts shouldn't be shared, so I don't think there currently exists a way to connect accounts to indicate they are family.

What you can do is get just Entry level subscriptions so you are spending a bit less on each subscription, or you can get 6 month subscriptions which on a monthly basis is the same as the Entry level cost, but gives you full access to 8 characters on each world.