Ask the program before deciding. Under the framework as checked, each joining spouse or child adds a fee and a gift, and the immigrant stage treats unmarried children under 21 as derivatives. Whether members can be added later is a question to put in writing.
Each added member is a cost and an age question
Hypothetical example: the children are 14 and 20. The younger child is clearly a derivative at the immigrant stage for years to come. The 20-year-old is close to the age limit for derivative treatment, and while the Child Status Protection Act can protect age during part of the immigrant process, it depends on petition and visa-availability timing that the family cannot control.
Including the older child from the outset costs an additional fee and gift under the terms as checked; excluding him may leave him needing his own route later. Ask the program two questions in writing: whether a family member can join after the principal is vetted, and whether any portion of a family member's fee is returned if that member is refused. Ask counsel for an age calculation under the CSPA for the older child.
Then decide with numbers, not with hope. The spouse's inclusion is not age-dependent but carries the same added cost.