Population censuses - UNECE
Child refers to a blood, step- or adopted son or daughter (regardless of age or marital status) who has usual residence in the household of at least one of the parents, and who has no partner or own child(ren) in the same household. Grandsons and granddaughters who have usual residence in the household of at least one grandparent while there are no parents present may also be included. Foster children are not included. A (grand)son or (grand)daughter who lives with a spouse, with a registered partner, with a consensual partner, or with one or more own children, is not considered to be a child. A child who alternates between two households (for instance after his or her parents have divorced) should consider the household where he or she spends the majority of the time as his or her place of usual residence. Where an equal amount of time is spent with both parents the place of usual residence should be the place where the child is found at the time on census.
United Nations Economic Commission for Europe (UN-ECE) (in cooperation with Eurostat), "Conference of European Statisticians Recommendations for the 2010 Censuses of Population and Housing", New York and Geneva, 2006