Don't worry about what you think the parents expect you to do with their children during the day. You have lots of kids to look after during the day and they don't get it. There is nothing wrong with free play. My daughter's government regulated pre-school had this on their facebook page and I found it really interesting:
Free play is the 'meat and potatoes' of our program. A certain amount of free play time is actually mandated by the government. It gives children the chance to choose for themselves those things that interest them. Naturally drawn to things from which they can learn, they can explore, discover, test theories, cooperate, use and develop skills in all areas of development.
I also have a very structured day but that includes lots of free play. The kids seem to enjoy it the most. I agree with 5 Little Monkey's - switch toys around.
Try and get some alone time in the evenings and weekends. The best thing for me I ever did was to hire a house cleaner bi-weekly for 2 hours. Just not having to do so much housework has helped.