The program states that each joining spouse and unmarried child under 21 must be considered in the initial application, so plan on naming them from the start. Travelling at different times is a logistics question; being included is an application question. Ask counsel before assuming anyone omitted at the outset can be brought in afterwards.
Treat inclusion and arrival as separate decisions
Write two lists: who will be part of the application, and who expects to travel when. The first list has to be settled before filing; the second can flex around schools, work and housing. Each named person still needs to meet the general requirements — eligibility for permanent residence, admissibility, and an available visa — because inclusion in a household application does not substitute for those.
Where a person's plans are unresolved, get advice on the options rather than guessing. Two points belong above both of those lists. The payment runs from the applicant to the U.S.
government and buys neither a classification nor relief from any ground of inadmissibility, so each named person's own record still has to bear examination and one person's difficulty can affect the household's plan. And a determination must still be made under EB-1 or EB-2, subject to visa availability, which means the question of what the principal's record could support is the substantive one and should be answered in writing before the household list is priced. Confirm current terms and amounts against official sources rather than a summary prepared for another purpose.
Hypothetical example: a parent intending to move first asks for a written view on which category their record could support and what would establish it, and receives a shorter and more useful document than any of the cost projections already prepared.