Praise be to Allah.
If a young child or a person who is insane finds something that has been lost, and picks it up, then his guardian should take on the responsibility of identifying its owner in his place. The guardian should take it because they are not qualified to be entrusted with it or to take care of it. If he leaves it in their hands and it gets damaged, he will be held responsible because he neglected it. If the guardian (of the child or insane person) announced it and tried to identify the owner, and nobody comes forward to claim it, then it belongs to the child or insane person, as is the case of the adult or sane person [who finds lost property]. For more details on the rulings pertaining to lost property see Question # 5049