Even though women and minors are exempt from machatzis hashekel (as discussed in yesterday’s post), they are permitted to donate if they so choose. The only catch is that the money must be donated to the public; a korban tzibur cannot be bought from money over which an individual retains private rights of ownership. The Minchas Chinuch quotes a question raised by the Sha’ar haMelech and Ktzos (235:4): We are familiar from the laws of sukkos with the din that a katan cannot make kinyanim. One should not give a lulav to a katan on the first day of Yom Tov (before one has fulfilled the mitzvah) because the lulav must be owned (“lachem”) and a katan can acquire the lulav but has no ability to be makneh the lulav back to its adult owner. By that same token, a katan should have no ability to be makneh his own machatzis hashekel to the public!
Perhaps the simplest solution to this question (which the Minchas Chinuch rejects) is to accept that the kinyan of the katan is in fact invalid, but considering that there are thousands of other shekalim in the public store, the few shekalim contributed by ketanim are bateil b’rov. Another possible approach stems from the fact that the Rishonim debate whether the obligation of machatzis hashekel begins from age 13 or from age 20. Perhaps the ketanim spoken of are halachic adults above the age of 13 who have not yet reached the age of 20. The Ketzos denies the whole premis of the question and uses this problem to bolster his contention that a mitzvah d’oraysa can be fulfilled using kinyanim derabbanan. One of the classic cases of this genre is the question of whether kiddushin using a ring purchased though a kinyan derabbanan has any validity on a d’orasya level.