Each eligible unmarried child under twenty-one who joins adds a further US$15,000 processing fee and a further US$1 million gift, on the published figures, and the same applies to an eligible joining spouse. Those amounts should be re-verified officially before any payment.
Per person means per person, and one of them has a birthday
The structure does not treat a household as a unit, so the cost of including a family rises in a straight line with its size. For Thaddeus that is his wife and both children, each adding the same processing fee and the same gift amount. What deserves separate thought is the sixteen-year-old.
Eligibility as a joining child depends on being unmarried and under twenty-one, and both of those facts can change while an application is in progress. Nothing about paying for a family member creates status for that person; each is assessed on their own eligibility and admissibility, and because successful applicants proceed through EB-1 or EB-2, the family's outcome ultimately depends on that classification's rules and on visa availability rather than on the payment schedule.