Pulling a training out of thin air is not an easy task, so I can understand that you are a bit nervous about that. What is unclear in your case is this: what is the assignment you have been given, what is the goal of the training?
Obviously, it's not about teaching them how to supply good service. Is it to instill in them a service culture that they will then take and spread in their branch offices?
If this is the case, you may want to think about what is driving them. For instance, if they are motivated by increasing the bottom line (most managers are, because of fat bonusses) you may want to work with them through cases and best practices that have proven to increase the bottom line. If you can convince them how good customer service helps them reach their goals, you're more than half way there.
Maybe you should just ask them what goals they are looking to meet, and how they think customer service may help them achieve those goals.
Also, you're saying that your senior management is forcing to attend your training, because they want to drive a good service culture (good for them!). You have there a great sponsor! Make sure you get him/them/one of them to tell your group about why they want this.
Just some off-hand remarks. Maybe you're already getting some ideas of this. Will be interesting to see what others come up with! :-)
At your service,