Give Them Their Own Bank Account
Your middle school child is old enough to have and to manage their own bank account. Start them off with a small amount of money that they are able to invest, and give them the opportunity to save and take charge of their own finances. Encourage them to deposit their own money from that point on, and remind them to keep track of how much interest they earn. It will certainly be a learning experience for them that will start them off on the road to future savings.