That is a tough situation. I stay at home with my girls and feel like I run out of energy around 10am. I think it is great that you are willing to get your son invloved in something else after you come home. I am sure you would rather just put on a movie for them and rest yourself.
I would suggest looking for some other moms in your area in the same situation. If you don't find any here, you might try yahoo groups. If you could schedule a playdate with some other parents, the kids will entertain themselves while you hang out and socialize a bit. That has been a great way for me to breathe at times and let the kids have a blast too.
I am sure that money is tight for you, but have you thought about hiring a student to come and hang out with your kids after you get home? Since you will be there, it might not be as stressful for your kids or yourself and you might not have to pay as much. That would give you some freedom to do some things around the house or ust relax a bit and give your kids someone with a lot of energy to play with. You could even have certain educational activites ready for them to play. Just a thought.
Another thought - we have a Monkey Joes that I like to take my girls to every once in a while. I can take both of them five times for what I pay for a month of my daughter's gym class. They can play as long as they want and there is a wonderful parent area (and baby play area) for relaxing or getting work done (wi-fi). My oldest always makes new friends and sleeps great that night.
There are also different classes available at recreation centers that might be a little less expensive. We really like pottery classes.
Hope you find something wonderful that works for you!