Civil documents establishing the marriage and each child's birth, and each child's age at the relevant stages. Derivatives are the spouse and unmarried children under twenty-one.
Dates of birth are load-bearing facts
Hypothetical example: The documentary requirements are ordinary but the ages are consequential, because derivative eligibility for children generally ends at twenty-one and the process runs for years. A household should record each child's date of birth against the expected stages and identify where an age issue would arise. The Child Status Protection Act contains provisions that can affect how age is calculated in particular circumstances, and visa availability as reflected in the Visa Bulletin influences the timeline that drives it, so the analysis is specific to the facts and should be taken as such rather than assumed.
Civil documents themselves should be obtained in certified form early, particularly where any were issued outside Canada, since replacement copies can take months. Any discrepancy in names across documents should be reconciled with whatever record explains it.