If your children are young, let them enjoy having a pet to love without worrying about the fact that its lifespan may not be forever. If you do, they will learn about responsible pet ownership in good time, as well as the responsibility that comes when the pet becomes so old or sick that it can no longer enjoy life. While death is a part of life, I don’t think that reality should be impressed upon your children now.
I can’t think of a polite way to tell someone you’re afraid your family members wouldn’t be safe around him, and I don’t think it will be necessary because Darrell is going to get the picture without anything being said.
It’s regrettable that your co-worker didn’t have his day in court because at the office it appears he has already been found guilty. It goes without saying that you have to find a lunch partner and mentor besides Darrell, so be prepared.