According to the US immigration law children are usually included in their parents’ petition process. Children derive the ability to immigrate from their parents, without the need to apply separately. So if you are a US citizen or a lawful permanent resident who wants to bring a relative to the USA on a family preference visa, you don’t need to file separate I-130 petition for any children your relative has. You can just list names of the children in Form I-130 that you are filing for your relative.
Part 4 “Information about beneficiary (continued)” of Form I-130 contains section “Information about beneficiary’s family” where you must provide information about the spouse and all of your relative’s children. There are fields for 5 persons, where you must indicate first, last and middle name, relationship, date of birth and country of birth for all beneficiary’s children and for his/her spouse.