Is it considered backbiting for a young child (two years old) to say that he would hit his mother's brother-in-law (the husband's brother, who is the mother's sister's husband) (in his absence)? And is it considered backbiting if I hear backbiting about a specific person whose name is mentioned but whose appearance I do not know?

A person should not speak unless the speech is good, or its benefit is evident. The default is to refrain from permissible speech for fear that it might lead to something forbidden. Forbidden backbiting is when a person mentions about his brother something he dislikes, such as flaws or the like. Whoever hears forbidden backbiting must rebuke the one who utters it. If he is unable or his rebuke is not accepted, he should leave that gathering if possible; otherwise, he becomes a backbiter himself.