Dependants hold L-2 status in reliance on the principal's status, so their position is affected when the principal's ends. A spouse working in valid L-2S status therefore has a second income tied to the same date, and children's schooling arrangements sit on that same calendar.
Plan the household transition too
A family several years into an assignment has commitments that cannot be unwound quickly. Set lease terms, school years and a spouse's notice period against the constraining date. Where a child is approaching 21, raise that separately with counsel, since dependant eligibility is limited to unmarried children under that age.
The household's timetable is often the one needing the longest lead. Set the household's fixed commitments against the constraining date on a single sheet, since a family several years into an assignment usually has more of them than it realises: a lease with a break clause, a school year, a spouse's notice period, a car finance agreement and possibly a mortgage. Each has its own notice requirement, and the longest of those is what determines when the household must be told.
Where a child is approaching twenty-one, raise that separately with counsel, since dependent eligibility is limited to unmarried children under that age and the two dates may not be the same one.