Civil documents proving each relationship, plus each dependant's own identity records. L-2 spouses are employment-authorized incident to status; L-2 children may study but may not work.
Relationship proof, then the spouse's own file
Hypothetical example: The dependants' documentary needs are modest: a marriage certificate, birth certificates for children, and identity documents, with any name discrepancies reconciled by the document that explains them. What deserves more attention is the spouse's own position. Because an L-2 spouse is employment-authorized incident to status, the spouse can plan to work without a separate sponsor, and employers will nonetheless want documentation they can record, so the practical form that takes should be confirmed for the specific case.
If the spouse holds a regulated occupation, a second file is needed entirely: the destination state's licensing body will have its own evidentiary requirements, often including verification sent directly from the Ontario regulator. That request should be made early, because it is routinely the longest item in the household's whole plan.