Mishnah Shevuot 6-4

We don't swear in response to a claim from somebody who is a cheresh, a deaf-mute, a shoteh, somebody who lacks knowledge, or a katan, a child, a minor. These people are considered to be lacking in intelligence, and therefore, there is no requirement to swear in response to a claim they make. Also, we don't make a child swear anything, whether it's a de'oraita (biblical) swear or a derabanan (rabbinic) swear. They don't understand the seriousness of swearing. Children don't understand that, so we are not going to obligate them in any situation to swear. But one swears to a minor before collecting from his possessions. If there's a unique scenario where a person has to collect from a child, no matter what the situation is, Hachamim (the sages) enacted that a person has to swear that it's his item in order to stop people from taking advantage of children. A person collecting a debt from a child is obligated to swear. And one swears to the supervisor of Hekdesh, of the treasury of the Beit HaMikdash (the Holy Temple), before collecting from its possessions.