Am I right in thinking that a Death Certificate is the best place to get the most info. - like in Australia? I do wish British Certificates were so informative!!
New Zealand Death certificates are not quite as detailed as those from some Australian States. For example, (in NZ) ONLY the "living issue" of the deceased person is recorded; children's names are NOT shown (they're recorded only as male or female and age).
If you are only wanting to determine whether the couple in question had children, then a death record is the obvious choice, but it should be borne in mind that other information contained, may not be wholly accurate, supplied as it is, by a third party (the informant to the death).
The death record for Rosina will also likely only state "when/where and to whom, married" and note also that there is "no widower" :
The marriage record (1907) would contain details of the parents of both bride and groom, and is of course information supplied by the parties themselves. "Printouts" (images of actual registers), rather than certificates, is the recommended purchase for all NZ BDM records.
To learn what information is contained in records, refer to the Resources section of this board >> "BDM Information" :