It's possible that you need to adjust your dosage (and/or timing) of Mestinon and/or start on another drug. Certainly stress - a new job, and teenaged kids - will not help you any!

Have you ever been tested for Celiac Disease? It's possible that you may have a problem with gluten. Many people with MG find that a gluten-free diet can do wonders for their energy (I had a CD diagnosis for 2 years before my MG diagnosis).

Here's a link to a post with more information about Celiac Disease/Gluten Sensitivity:
http://neurotalk.psychcentral.com/post8767-2.html

If you are comfortable with your boss/employer, then by all means bring up the subject to them. It is possible that there may be another position in the company that you would be able to do that wouldn't tire you out as much. Part-time might be a good option, as you wouldn't get as tired throughout the day. I don't know where you work, but is it possible to do any of the work from home? That might make it easier for you as well, and as long as you are being productive, your employer would probably be willing to let you do a trial period to see how it goes.

It is so vitally important that you conserve your energy as much as possible; otherwise, you could seriously damage your long-term health! But I'm guessing you already knew that.....