What is source for covering head of son with your tallit during birchat cohanim?

Shalom, Thank you for your question. When the Temple was standing, and the cohanim blessed the people, the shechina (Devine presence) rested on their hands, and it was forbidden to look at it directly. Today, unfortunately, this level of shechina does not occur, and from the letter of the law it is permitted to see the hands of the cohanim. Even today though, we refrain from staring into the cohen's face so as not to distract him from his blessing. However, the general custom is for the cohanim to cover their faces with their tallit, so from the letter of the law it would not be problematic to look at the cohanim during their blessing, as they would not see us staring at them. The custom however is to continue the practice from the Temple times and refrain from looking directly at the cohanim even if they cover themselves with their tallit. (See Shulchan Aruch 128,23 and the Mishna Brurah there). The custom for the congregants to cover themselves with a tallit, and to cover the heads of their children, can be found in the commentary of the Rema, (Darkay Moshe, Orach Haim, 128, 23, (15)). He records the custom of his country (Polland), for the congregants to also cover themselves with their tallit so as to avoid looking at the hands of the cohanim. I assume that in communities where the boys didn't wear a tallit (or at least not on their heads), they kept this custom by going under the tallit of their father. I hope this is of a help to you. Blessings.