Each number transferred will need a separate account and email address. As for your two children, as long as you use an email address for each of their accounts you would be in charge over it. The email address used for their account doesn't have to be their email address but one you generate for the account.
Just to point out, Visible does not have family accounts, they are single line accounts. The base plan, even though it being unlimited everything, is deprioritized when it comes to data. If you go the plus plan route on all four phones I would think Verizon would be the better option for multiple lines unless you are looking for truly unlimited data.
