Metallic taste, isolated when in living room, kids have dark circles under eyes, not gas leak or carbon monoxide - any other possible environmental?

Possibly radon. I would have your home checked for radon, especially in basements or crawlspace under living room. Metallic taste and fatigue are symptoms associated with radon exposure. Check with your local public health department about how to test for radon.
My own opinion: If i were in your position i would do two things- first, call the epa, and find out if any government agencies test homes for possible pollutants or toxins. Two- do a google search for companies in your area for contractors who are licensed and/or certified to handle household toxins, and can prove their qualifications for your specific need. Three-see a physician for an evaluation.