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Should I charge my children interest when they borrow money? 
Tell them you will not loan them any more money until their existing debts are paid off. This is closer to how the real world works and it won't come across as vengeful or like your changing your initial "contract". If they protest, lovingly tell them that your money is not their money, and that an interest free loan from their father is a privilege, not a right. As far as charging interest on your loans, go for it! Charge them 5% or something small. Just don't do it on the existing loans or that will come across as changing your initial "contract" again, and perhaps once they've proven themselves to be reliable borrowers they can once again earn the privilege to have an interest free loan. The book "The Millionaire Next Door" has really good thoughts on this in its section on Economic Outpatient Care.