Reuven died, leaving a widow (Sarah) and her children and a daughter (Dina) from a previous wife (Leah). Dina presented an old document stating that Reuven, at the request of his late wife Leah, was giving a present to Dina of certain property, to take effect immediately, but that the actual control of the property would be transferred only when she was getting married, and only if Reuven did not otherwise arrange a proper dowry for her. Sarah claims that the present was invalid because, among other reasons, Dina had been too young at the time to take part in a transaction.

The mishna (Bava Batra 156b) says that it is possible for others to acquire something on behalf (zachin) of a katan (minor). The Rashbam explains that since the katan is not capable of doing a kinyan himself, they made a takana that others can do so on his behalf. The implication is that if the katan was old enough to make his own kinyan, others would not be able to make a kinyan on his behalf.
The Rambam (Mechira 29:11) says that when a katan pays money for a field and makes a kinyan on it, he acquires it. The Maggid Mishneh says that the source is the fact that the leaders at the time of the division of Eretz Yisrael acquired land on behalf of the children (Kiddushin 42a). This is a puzzling connection because the Rambam is talking about a child who does the transaction himself, when he has some understanding, and the precedent is talking about an adult doing the transaction on the katan’s behalf even when he is an infant. Apparently the idea is that the reason the transaction worked for the katan is that there was involvement of the adult seller, which is equivalent to an adult doing zachin for the child, and that which the Rambam/ Maggid Mishneh were learning was that this works even to acquire land. The Rambam holds, as opposed to the Rashbam above, that zachin works even on the level of Torah law.
While he raises complications regarding the different opinions of the Tannaim as to whether there is zachin for both an adult and a child, the Hagahot Oshri says that is common practice to be "koneh" on behalf of a katan, and that is a reference to a kinyan sudar (transfer using a utensil such as a handkerchief).
In our case, the document states that the witnesses did a kinyan sudar on behalf of Dina. The term "v’kanina" indicates that they did it in the form of zachin. The Ba’al Ha’itur posits that this works even if the kli used did not belong to the person who was receiving the object, as is clear from Tosafot (Kiddushin 26b) in regards to doing a kinyan on behalf of one who is not present. The gemara (51a) says that the one requirement is that the recipient must be "in the world" (i.e., already born), indicating that even an infant who was just born would acquire by others’ actions. Tosafot (Gittin 65a) says that an older child can make certain kinyanim, but not sudar, again implying that when others do the kinyan for him, it works even through sudar.
Therefore, Sarah cannot disqualify the present that Reuven gave his daughter Dina in his lifetime based on Dina’s age at the time of the transaction.