Unmarried children under 21 may seek L-2 as dependants but are not work authorized by that status, so an older teenager who expects to work needs a separate discussion. Childcare costs also change the arithmetic behind the date the second income becomes necessary.
Cost the gap, not just the move
A household relying on two incomes usually faces a period with one. Estimate its length honestly, add childcare that may be needed once the spouse is working again, and check what coverage each family member has in the interval. School enrolment timing often forces the family's hand more than any immigration date does.
Put those figures beside the immigration plan so both are discussed together rather than in separate conversations. Set out the childcare arithmetic beside the income arithmetic, since the two are frequently discussed in separate conversations and only make sense together. A second salary that arrives with a full-time care cost attached is a different proposition from one that does not, and in some households the honest conclusion is that the spouse's return to work should wait for a school place rather than precede it.
Note also that an older teenager in dependent status is not work authorized by it, so a family assuming a summer job as part of the plan needs a separate answer for that person.