you'll send a cover letter, an updated I-130, a copy of a notice of receipt/approval for the original I-130, and copies of marriage/birth certificates to whereever the application is at that moment. If I-130 has been approved, you don't need the updated I-130, just the supporting docs and the explanation.
Make sure that, when the time comes, DS-230 forms are filled out for every one of the derivative family members.