Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Should I charge my children interest when they borrow money? 
Because this is Money.SE and you're connecting it to offspring, I'd think about a discussion with them to get their agreements. From my perspective, anything (my wife and) I have will go to offspring in the end. As such, everything borrowed and not repaid simply reduces the estate by that much. Among multiple offspring, such reductions should be against the borrower rather than spreading it out. That should be accounted for in whatever will is created. This would be the discussion point. It might also be discussed how or even if any interest should accrue for unpaid amounts. If, for example, a 1% APR is agreed upon for unpaid loans, then the final principle+interest amount is taken off of the borrower's inheritance. Existing outstanding loans might (or might not!) be useful examples for sample calculations if desired or needed. (If nothing else, they might serve as reminders that loans were not forgotten.) By having such a discussion, you can show that you are trying to plan for a fair distribution of your estate, perhaps thereby sidestepping any concern about charging interest to offspring for repaid loans. At the same time, you're handing over some financial responsibility, giving them a power of personal choice, which seems to be a part of what you're concerned about. Once such a discussion is started, it's possible that any question of interest will resolve itself naturally. The discussion almost necessarily must include all offspring at once. One will find it harder to negotiate from a standpoint of pure self-interest without objection from another. Think beforehand about what will be said and about what responses might come. Think things through as much as you can.