I own a business. If an employee breaks something during their shift, it's NOT their responsibility to replace it. Things get broken. Sh*t happens. Move on from it. She should have told you. I wonder why she didn't? Could it be that you're either not easy to talk to, or maybe a bit hypercritical? Check yourself. If she makes a habit of being spacy, THEN you have reason to fire her, as she could pose a safety risk to your kids, but over a microwave? Really?? (Eye roll).