Number of children under the age of 18 that live in a family in the interview month in which marital status is first observed (for those without spouses during the course of the SIPP) or in which the respondent's spouse is assigned. This number is the same for all family members and does not indicate that the children are related to a particular individual (fnkids for 1990-1993 panels, rfnkids for 1996-2004 panels).