Learn French: Le garçon a pleuré et il leur a avoué sa faute. - The boy cried and he confessed his mistake to them.

In the passé composé, indirect object pronouns like leur (to them) must go immediately before the helping verb (the auxiliary). Here, the helping verb is a, so we say il leur a avoué. You never put the pronoun after the verb or between the two verb parts.

Why do we use leur here instead of les?

Les is a direct object meaning them, while leur is an indirect object meaning to them. In French, you confess something to someone (avouer à quelqu'un). Because of that relationship, we need the indirect object pronoun leur.

The boy is masculine, so why is it sa faute and not son faute?

In French, possessive words agree with the grammatical gender of the thing being owned, not the person who owns it. Because faute (mistake) is a feminine word, you must use the feminine sa, even though the mistake belongs to a boy.

Does faute mean mistake or fault?

It can mean both! While faute can mean fault (as in being to blame for something), it is also the standard French word for a mistake or an error. For example, a spelling mistake is une faute d'orthographe.