We have a similar situation and I don't have a complete answer but might get you in at least one direction. Notice in settings you can have different email addresses or phone numbers for iCloud, iTunes, and messaging. We have one iTunes account for our music and applications (so most devices have this as the iTunes account). Then you can set up an iCloud account for each family member who is 13 or older. Use those accounts for iCloud storage and messaging. I'm not sure how that helps the younger people in your family but hopefully this gets you going a bit. Maybe if they have an other email account or phone number that could work.