AFAIK, Yibum requires a kinyan, so why not da'as?
Let me back up. M'doraysa, maaseh katan aino maaseh. If a 9 year old does yibum, the status of the yavam and yevama is the same as when a gadol does ma'amar. The whole thing is m'drabanan and they're sort of in limbo until they do yibum when he is a gadol.
As far as yibum being a kinyan, it's not the same kinyan as kiddushin. We certainly see that you don't need da'as from the cases of ones and shogeg, where the yibum is valid even though there's no da'as kinyan. The same thing is true by a katan, where despite the fact that he can't have da'as kinyan (at least in the classic sense) the rabanan considered it a valid yibum to a certain extent.