Plan a separate route for that child before the move, not afterwards. Derivative status for an unmarried child ends at 21, the treaty-investor category offers no direct path to permanent residence that would carry the family through, and a student status has its own admission and funding requirements that take months to arrange.
The birthday is fixed; the alternatives take time to build
Hypothetical example: Ulric and Blanche are buying an American sign-and-graphics fabrication business. Blanche intends to work in it, and their nineteen-year-old wants to attend college in the United States. Two answers are straightforward.
As the spouse of a treaty investor, Blanche is employment-authorized incident to that status, so she may work in the business or elsewhere. The child may accompany them as an unmarried derivative while under 21, and may study in that status. The third answer is the one that needs work: the derivative ends at 21, and because this category has no direct path to permanent residence, nothing in the parents' file will carry the child forward.
If the intention is a full degree, a student status of her own is usually the sensible route, and it requires an admission, evidence of funding and a separate application. Starting that eighteen months ahead is comfortable; starting it eight weeks ahead is not.