Dear Jingyi
there are different ways to identify the gender of respondent's non co-resident children, and which way to pursue will depend on your specific analysis (on which we cannot advise). The most comprehensive data on parenthood is collected in the fertility history module in Wave 1, see https://www.understandingsociety.ac.uk/documentation/mainstage/dataset-documentation/wave/1/questionnaire-module/fertilityhistory_w1. In particular, the variable a_lchsx records the gender of non co-resident children, see https://www.understandingsociety.ac.uk/documentation/mainstage/dataset-documentation/wave/1/datafile/a_natchild/variable/a_lchsx. For currently co-resident natural children, the variable a_lchno reports the current wave person number of the child (i.e. the child's a_pno), see https://www.understandingsociety.ac.uk/documentation/mainstage/dataset-documentation/wave/1/datafile/a_natchild/variable/a_lchno, and this variable can be used to compute the gender of the child from the _indall data (which has basic information for all individuals listed as members of the household).
Currently co-resident children may move out of the home later, or currently non-resident children may join the household later on in the life of the Study; if these are also of interest to you, you can track natural children and their gender from the _indall data files. For every person that lives with their biological parent, the variable "fpid" and "mpid" contain the cross-wave person identifier ("pidp") of the biological father/mother.
Note that the natural children identified through co-residence may be quite different from the natural children identified via the fertility history module in Wave 1: from Wave 2 onwards we do not know the gender of non co-resident children who are never observed as their parent's co-residents. And for individuals who joined the Study after Wave 1 (as temporary sample members) we do not collect information about the gender of their natural children.
I hope this answers your question.
Best wishes,
Gundi
PS. We provide examples of how to reshape and prepare the data for merging different data files in Understanding Society in our online training course materials. See https://www.understandingsociety.ac.uk/documentation/training/online
PPS. Note that a number of inconsistencies relating to co-resident children have been identified with the fertility history file in Wave 1. This includes duplication of data for some children as well as mismatches with pnos of children enumerated in _indall. See, e.g., https://www.understandingsociety.ac.uk/support/issues/155, or search all issues reported here on "fertility history".