A Matter of Interest: The Two-for-One Trade
The little girl is crying inconsolably, frustrated in her attempts to win a candy from her elder brother’s stash. Their father tries to intervene and make peace (and give the whole family a rest from ear-piercing screams!): “Teiere kind (my dear child),†he whispers to his seven year old son, “Give your little sister a candy. Tomorrow, iy’H, I’ll buy you two in return.â€
The Gemara’s warning is recounted in halachah: A person should not lend or borrow from their family members or children with ribbis (interest), “Dilma asi l’misrach†(lest they habituate themselves [to this practice]) chas v’shalom. Assuming the candy belongs to his son, and the father borrows it from him to give it to his sister, the payback of two candies can would be a form of ribbis.
It can be argued that the candy does not belong to the seven-year-old in the first place; a child, and certainly a minor, who is samuch al shulchan aviv (lit. trans. leans on his father’s table, i.e., is supported by his parents) doesn’t own anything outright,* and therefore the father is not actually borrowing the candy from his son, and the two replacements are not repayment of a loan, since all the candy belongs exclusively to the parent—so how can ribbis be involved?
Even if the candy that had been gifted to the older sibling is determined halachically to belong to the father, parents should not make such deals with their children. This is precisely what Chazal forbid: giving children a ta’am ribbis (any whiff of interest), dilma asi l’misrach.
An appropriate course of action would be for the father to buy the candy from the seven year old for the price of two candies, and pay him right away. Alternatively, he can turn this into an opportunity for a learning experience by saying to his son, "Please lend a candy to your little sister and Tatty will pay you back, plus an extra candy just for you." This way, the loan is made to the sister and the father pays the interest as a non-brokered third party. When ribbis is paid from someone other than the borrower, it is permitted, so long as the borrower doesn't state “so-and-so will pay you on my behalf†and the third party pays the extra as a gift without expectation of reimbursement from the borrower.
*See Halachah #539, Halachah #663 and Halachah #725 for more examples of children’s rights to possession.