Each accompanying spouse and each unmarried child under 21 adds a further processing fee and a further gift of US$1 million. The cost therefore scales with the size of the household rather than being a single family price, and eligibility for a child depends on being unmarried and under 21.
Multiply, then look at the birthdays
Families frequently model this route on the assumption that one payment covers everyone, and the arithmetic changes sharply once it does not. A couple with two children under 21 is looking at four processing fees and, on the individual route, the principal's million plus a further million for each accompanying family member. Whatever the household's means, that figure deserves to be established before any planning is done around it.
The age condition then needs a calendar. A child who is unmarried and under 21 may accompany; a child who passes that birthday, or marries, falls outside the description. Because the route runs through existing immigrant classifications, the timing of each stage can affect where a nearly-adult child stands, which is a question for advice on the specific facts rather than for a general rule.
Verify the current family terms from the official source before relying on any figure here.