Going from personal experience, my parents let myself and my brother and me borrow money from them all the time. However there was always some noteworthy things to take into account.
- I was never charged interest, but I was expected to pay it back in full ASAP. Yes, this gets delayed sometimes (e.g. the high interest car payments/mortgage/student loan took priority). However as soon as I had extra money, it immediately went to pay back parents. As a parent I would recommend this.
- The reason to borrow from your parents and not from someone else is explicitly because it is interest free. Otherwise there would be very little reason to ask family for help.
- You can always use this as leverage in future events. Child getting married? Here is a $3000 wedding gift to cancel your loan.
As an example, I borrowed a large sum of money on my student loan (we will just say it was $50,000). I had saved nearly $30,000 on my own and my parents lent me $10,000. I paid the remaining off over the course of about a year and a half. After this loan was paid off - I started paying my parents back.
They dictated that I should not worry about paying them until my other interest loans were paid off. Once they were, my priority was to pay back my parents.
Its supposed to help your children get ahead a little bit rather then sucking out interest from them. As long as the money was not needed elsewhere and is spent on something important I would not worry about it. Just make sure they are aware they are expected to pay it back in a reasonable amount of time or with specific requirements (such as after other loans are paid off).