The government cannot require that interest be charged.  In fact, during the Middle Ages, the government prohibited interest from being charged, so the major banking families had to make money only on "late fees."  What the government can do is require interest-free loans to be considered as having a gift element if there is no interest stated above a certain rate.  You can calculate that amount and then figure out whether, together with other gifts to the same individual, the amount involved rises to the level at which a gift tax return is required.  

See also, "the parents that can't afford to give away money to their kids often do, and the parents who can afford to give away money to their kids seldom do."