One important thing that you need to teach your children about is the responsibility that comes through entering a contract for a service. Some common examples that come to mind are cell phone contracts, and gym memberships. One of the best ways to teach them this responsibility is to let them experience it. You can do this in a few ways.
1) You can cosign on many of the contracts with them. You may want to set up conditions, since this may affect your credit report as well. You may have them pay you the money each month, and then you in turn pay the cell phone company or the gym membership. You can save money by adding them to a family plan this way. If they are late you can still pay the obligation, but you take away the cell phone or gym membership until they are current. You should also charge a late fee.
2) You may want to encourage your children to choose options that allow them to pay as they go. There are many cell phone plans that allow people to pay as they go. In fact many of these programs were designed with the teens in mind. The price per minute is higher, but it may be worth not having the monthly obligation. It will also teach them discipline, since they will be responsible for topping up the phone each month.
In either of these options it is important that you place the responsibility of meeting their obligations fully on your children. This teaches true responsibility. It is also important for them to learn at an early age how late fees and penalties work. If you can teach them this in an environment, which you control, you can save them a lot of money and headaches down the road.