Welcome in the community! Unfortunately you can't add more than 5 people (+Admin) into a family subscription. But you can suggest the idea to raise the limit of the family premium subscription here. Probably there are also other people, which will support the idea ;).
I think for now the best solution for you and your family might be to have two family subscription. One with you as Admin and five of your kids and one with your wife as admin and the rest of your kids.