I plan to give Matanos l’Evyonim on Purim (by credit card payment) on behalf of my wife and children (over the age of bar/bat-mitzvah). Do they need to make a kinyan or is it enough to inform the person distributing the money that he should be zoche in the money on behalf of my wife and kids.
There is no need for the gabbai to be zoche in the money for one’s wife. However, he should be zoche in the money for one’s children over the age of bar/bat mitzvah.
See Responsa in Chut Shani (Shabbos vol. 3). The reasoning behind this is that we find with regard to sacrifices that a person can bring his wife’s offerings, Rambam explaining the reason for this being that ishto kegufo. For children, however, the money should be theirs, and the gabbai should therefore be zoche in the money for them. Note that according to the Steipler, children (even over bar mitzvah) who are supported at their father’s table, and have no income of their own, are exempt from the mitzvah. However, other poskim have not stated this chiddush.