Not Testifying on a Minor / Eidus L'Kiyum HaDavar

The Rambam in Hilchos Gerushin 6:9 says that a ketanah cannot make a shliach l'kabbalah to receive her get because appointing a shliach l'kabbalah needs witnesses and "we don't testify on a katan". The obvious question is why not give a simpler reason - a katan can never appoint a shliach because he/she has no daas?

R' Chaim on the Rambam answers that even though a ketanah can't appoint a shliach, she can have a shliach through zachin l'adam shelo b'fanav (assuming a scenario that the divorce was a zechus). Therefore, the Rambam said that even with zachin she cannot have a shliach l'kabbalah because in order to appoint a shliach l'kabbalah there must be eidim present and "we don't testify on a katan".

What does it mean that one needs eidim to appoint a shliach l'kabbalah. R' Chaim further explains that there are two types of eidim. First, there are eidim l'birur hadavar. These are eidim that simply tell us what happened. For example, they may inform us that someone was mechalel shabbos or stole.

There is another type of eidus, though, called eidus l'kiyum hadavar. For example, by a marriage or divorce eidim are needed at the event to make it a legal marriage or divorce. They are not only telling us what happened, they are part of what allows it to happen. By a shliach l'kabbalah there is a requirement of eidus l'kiyum hadavar, whereas there is no such requirement by a shliach l'holacha (the shliach who deliver the get or kiddushin on behalf of the husband).

In Hilchos Yibum V'Chalitzah 4:16 Rav Chaim further explains the idea of needing eidus l'kiyum hadavar. He explains that for some "halachic events" all that is required to create the challos is the maaseh. [For example, by shechitah you just need to do the maaseh shechitah and the animal is shechted.] Other times the challos is created by the daas of the person doing the maaseh. [For example by divorce the husband is really using his daas to divorce the wife - it isn't just purely a maaseh.]

Sometimes the line is blurred in this. For example, by chalitzah Rav Chaim says that the challos is really affected purely by the maaseh. However, part of the maaseh chalitzah is kavanah. This is not the same as saying that daas creates the challos. The difference is that by chalitzah if a katan does a chalitzah with a gadol instructing him what kavanah to have - it works. This is true even though a katan has no daas. The idea is that the kavanah is just a part of the maaseh chalitzah - but the maaseh creates the challos - not the daas.

Therefore, Rav Chaim explains that by gerushin and kiddushin where the daas baalim creates the challos - there is a rule that eidim must be present also for the challos. The requirement for eidim is directly connected to the requirement of daas. This is specifically true by dvarim shebervah because of the rule of ein davar shebervah pachos mishnayim. By other areas of daas we don't necessarily require eidus l'kiyum hadavar - for example by appointing a stam shliach.

Apparently by appointing a shliach l'kabbalah there is an involvement of daas that is considered to be davar shebervah. However, by a shliach l'holacha it's just a regular shliach in kol hatorah kulah and there is no need for eidus l'kiyum hadavar.