Parshas Pekudei/Shekalim:Katan Donating Shekalim

The Mishna in Shekalim (1:3) says that a katan is patur from donating a 1/2 shekel to teh beis HaMikdash. The Bartenura says it is referring to someone under 20 years old. This is the Chinuch's opinion as well. Tosafos Yom Tov argues and says it refers to someone under 13. Anyone over 13 must contribute. The Rambam (Hilchos Shekalim) and the Ramban also hold anyone over 13 must contribute. The mishna also says that if a katan donates we accept it from him. The achronim ask how can we accept the money from a katan, m'd'oreisa he can't be makneh an object and therefore the money m'd'oreisa will never belong to hekdesh. It would then come out that the korbonos bought with the money are korbonos yachid and not korbonos tzibbur.

There are a few mehalchim to answer this question.

Sha'ar HaMelech

The Sha'ar HaMelech (Shekalim 1:5) writes that the coin the katan gives is batul with all the other coins. Even though Tosafos in Me'ilah says money is not batul, nevertheless this is only m'd'rabanan. The Rashba says that m'd'oreisa bitul applies to everything. Therefore, in our case the katan's kinyan works m'd'rabanan and on a d'oreisa level we can rely on bittul.

Avnei Miluim I

The Avnei Miluim (siman 28) argues on the Sha'ar HaMelech. He says that bittul does not work by money. The reason is because by money we are dealing with ownership and you can't be m'vateil someones ownership of something. (There is no bittul of teh cheftza like by issurim). The Avnei Miluim answers that at worst the coin the katan gave to hekdesh doesn't belong to hekdesh and hekdesh is stealing the coin. However, when hekdesh buys something with that coin whatever it bought belongs to hekdesh. For example, when you buy a lulav, there is no chiyuv to make sure the mony you use belongs to you. The reason is because if the money is not yours, the lulav becomes yours. Whether the money is yours or not is irrelevent. The same thing here and therefore the animal hekdesh buys is korbon hatzibbur.

(it is not so pashut whether the Avnei Miluim is correct in understanding bittul by money. The Bikurei Ya'akov on Sukka disagrees)

Avnei Miluim II/Ketzos

The Avnei Miluim (also found in the Ketzos) suggest a different answer. He raises the question whether a kinyan d'rabanan can work for a d'oreisa. (For example, ma'mad shalshtan is a kinyan d'rabanan and the question is raised whether you can use the object to marry a woman). He brings a rayah from our case that it works. The kinyan from the katan is only d'rabanan yet it works to make it a korbon tzibbur m'd'oreisa.