Their L-2 status ends when hers does, because it is derivative. The husband’s work authorization ends with it, and the son’s L-2 ends at 21 regardless. A multinational-manager immigrant petition can carry all three to permanent residence if filed in time.
The family’s status has no clock of its own
L-2 status is granted to the spouse and unmarried children under 21 of an L-1 principal for the same period as hers, and it cannot outlast her status. In the hypothetical, her husband works as a bookkeeper for a U.S. accounting firm on the strength of L-2 employment authorization incident to status; that authorization ends when her L-1A ends, whether at the seven-year cap or earlier, and his employer should know that the date is fixed.
Their son, who is 18, may study in L-2 status but will lose it on his twenty-first birthday even if her L-1A continues, at which point he needs a student visa or another status of his own. An immigrant petition for her as a multinational manager includes the husband and the son as derivatives if he is still under 21 when they seek to adjust, and the Child Status Protection Act may preserve his eligibility for the time the petition was pending. Filing that petition with a year or more of L-1A remaining, and adjusting before both the cap and the birthday, keeps the household together.