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One may NOT lend his children interest in order to them a lesson.
Bava Metziah 75a – Rav has such a hava minah, and the Gemara rejects it, because it will teach his children to lend with interest, realizing how much can be made (Rashi).
Yorah Deah 160,8 forbids it based on this Gemara, even if his ultimate intention is return the money to them as a gift (see the Taz).