On the Select Samples screen, you will need to choose “ATUS respondents and members of their households” in order to create your desired household characteristic variable. As well, you will need to include the variable AGE_CPS8 in your data extract. After creating your extract, you can use statistical software (e.g. SAS, STATA, SPSS) to create a variable that records the number of children under age 12 in each household.
You could also construct an “under age 12” version of HH_CHILD by following the above instructions and simply using AGE instead of AGE_CPS8. AGE was collected during the ATUS interview, while AGE_CPS8 was collected 2-5 months prior to the ATUS interview at the final CPS interview. Using AGE may be preferable for your purposes, since children born between the final CPS interview and the ATUS interview would not be recorded in AGE_CPS8.
Hope this helps.