If you book multiple rooms under one reservation number, which can only be done if you call Disney to make the reservation, then you should be able to unlock all of the rooms with your MagicBand or the unlock door feature. If each room has a separate reservation number than only the people who are in that room will be able to unlock the door with MBs or the unlock door feature.
You can make the reservations for each of your family members using your
MDE account. But if you think they will ever be going on WDW in the future, then they should have their own accounts.