Teaching children about sin

 The gemora Shabbos (149b) states that it is allowed to lend money to one's children with interest in order to give them the experience of usury

Shabbos (149b) One may lend to his sons and household on interest, in order to give them experience thereof.

Rashi explains that it is not actually usury  since the father owns the money lent and that used to payback

Rashi (Shabbos (149b) to give the experience of usury – so they will know how difficult it is. However since everything belongs to the father it is not actually a sin