The story is told of a five year-old girl who received a gold necklace as a gift from her uncle. The father, who had been experiencing financial difficulties, wanted to sell the necklace and use the money to pay his debts and help resolve the family's crisis. The mother, however, protested, arguing that the necklace belonged to the daughter and the father thus had no right to take it from her without her consent.
Dayan Shlomo Cohen addresses this question in his work "Pure Money" (vol. 1, p. 108) and cites in this context a ruling of the Rama (Rabbi Moshe Isserles, Poland, 1525-1572), in his glosses to the Hoshen Mishpat section of the Shulhan Aruch (270:2). The Rama there establishes that so long as a child is supported by his or her parents, anything the child acquires belongs to the parents. Hence, a father of a five year-old child indeed reserves the right to do what he pleases with the gifts received by the child, until the age of Bar-Mitzva. Once the child reaches Bar-Mitzva age, he is capable of owning property and thus the parent has no legal control over the child's belongings.
This Halacha does not apply to stocks purchased specifically for a child, or to a bank account that was opened on a child's name. For example, the question arose regarding a father who opened an account for one of his children and deposited a sum of money in the account each month. After the father's death, the other children demanded that the account be evenly distributed among them, since it was opened in the name of a child, whose money is actually the property of his father. This argument, however, was mistaken; since the father opened the account and deposited the money specifically on behalf of this child, the funds remain on his name and are not distributed evenly among the brothers.
Summary: If a child who is still supported by his parents receives a gift, it essentially belongs to the parents and they may do with it whatever they wish. Once a child reaches Bar-Mitzva, however, he legally owns his belongings and his parents enjoy no rights over them. If a parent deposits money in a child's account or purchases stock in a child's name, the money or stock belongs to the child, even though he is still a minor.
