Tosefta sobre Demai 7:9

[In contrast,] a chaver that was sitting [as a guest] at the wedding feast of an am ha'aretz -- and even if we see him take and eat food immediately, it is not presumed to have been tithed, as he may have tithed "in his heart" (i.e., by making a mental stipulation by giving tithes in exchange for what he ate when he returns home, see Dem. 7:1 Y. Dem. II.2.7). [But in the case that] his son was dining next to him, he [must] tithe for him. [But] if thee is someone else [dining next to him], he does not tithe for him. [And if] his son was sitting in another place [not next to his father, the father still] tithes for him.