As per the age of your son you mentions i would suggest Yes, charge them a interest amount but lesser than the market rate. And give them a valid reason behind taking interest on given amount. The reason you might grab from below real incident happen with me at the time of Diwali last year.
I am 26, and i am currently doing job and my salary is not so much that i can accomplish all my dreams of buying expensive Watch and many things. So i borrowed some strong amount from my mom.
She gave me the amount but she asked me to pay interest of 5% and when i asked the reason behind demanding the interest she said something which was valuable things.
She said me "If i would not give you money then you will definitely ask money from some money lenders or your friends because now that watch is your first priority. And in that case you need to pay the higher interest rate to them. And in life there might be situation where we would not capable to help you in terms of financial. So this is the time you should learn to pay interest and responsibility of borrowing amount and repaying it on time with interest rate. This will help you also to learn a lesson and our money will be withing home
I am not expert in parenting because i am still unmarried but i shared my point of view for your question.
Thanks