Short answer. You are bound by it. An adult who accepts a deposit from someone without capacity to contract takes on all the obligations of a keeper, and the item can be demanded back by the minor's guardian or administrator, or by the minor himself once he comes of age.
What the law says
If a person having capacity to contract accepts a deposit made by one who is incapacitated, the former shall be subject to all the obligations of a depositary, and may be compelled to return the thing by the guardian, or administrator, of the person who made the deposit, or by the latter himself if he should acquire capacity.
Civil Code, Article 1970 — Deposit by an Incapacitated Depositor. Read the full provision →
The incapacity protects the minor, not you
Rules on capacity exist to shield the young person from arrangements he is not equipped to judge. This article makes sure they are not turned around and used against him. Having taken the item, you carry every obligation a keeper carries: to look after it with the care of a reasonably prudent person, not to use it without permission, not to hand it to someone else for keeping unless that was agreed, and to give back the very thing when it is demanded. You cannot answer a demand for its return by pointing out that the person who handed it to you was too young to make a contract.
Who can validly demand it back
Three people can compel the return: the guardian, the administrator, or the minor himself once he has acquired capacity. That list is also your safety instruction. Returning a valuable item to a fourteen-year-old on his own say-so, or to an elder sibling who turns up asking for it, is not a discharge, and you may find yourself accounting for it a second time to the person actually entitled. Where the parents are separated or the family is in conflict, this is where keepers get caught. Ask who holds authority over the child's property before you hand anything over.
What the article does not settle
It does not tell you whether the child owned the item to begin with. A minor may be holding something belonging to a parent, a school or a friend, and a person with a better claim can pursue it regardless of your arrangement. It also does not authorise you to keep, sell, pledge or use the thing, or to apply it against money the family may owe you. And it does not make you an insurer of the item against every misfortune — you answer for the care you failed to take, not for every loss that could conceivably occur.
What to do now
Tell a parent or guardian, in writing, that the item is with you and that you are holding it for return. Describe it, date the note, and photograph the thing. If the deposit is an awkward one — money, a phone, jewellery, or something a child may have taken from home — say so plainly rather than waiting to be asked, and arrange the handover to the adult responsible for the child's property, against a signed receipt. Keepers who create a written trail at the start are almost never the ones who end up in an argument about what was left with them, or in what condition.