I am SO SO sorry, mama!
I agree with pp - don't tell the kids how the cat died. Go get a sensor for the garage door.
Also, try to be honest with yourself and DH about the blame you must feel towards him. No, it wasn't intentional, and I'm sure intellectually you know this and don't blame him. Emotion is another issue and just be honest with yourself so that you can get past it and deal with your grief.
Thoughts and prayers with you and your family (and...