Civil documents that establish each relationship, with certified translations where they are not in English, plus each child's date of birth checked against the twenty-first birthday and any earlier immigration history in the household.
Gather the civil papers before the corporate ones are finished
Corporate evidence dominates the conversation in an L-1A file, and family documents get left until the week before travel, which is when the missing certificate is discovered. Derivative status for the spouse and for unmarried children under twenty-one depends on proving the relationship, and the proof is ordinary civil registration: a marriage certificate from the issuing authority rather than a photograph of the ceremony, long-form birth certificates naming both parents, and the record of any name change including the one that happened at marriage. Translations must be complete and certified; a partial translation of a Québec certificate is a predictable delay.
Hypothetical example: a household leaving Saint-Joachim-de-Shefford discovers that a child's birth certificate carries a father's surname the passport does not, because of a change registered eleven years earlier and never carried across. Add earlier history to the same folder. Any prior U.S.
refusal, overstay or removal in any family member is relevant, is discoverable, and is better disclosed with an explanation than found later without one.