My sons and daughter in law all call me by my first name. I have known my daughter in law since she was seven and I used to work at her school, so it wasn't until she was a teenager and a regular visitor to our home that she felt comfortable using my given name. Her brother (who was actually in my class) still finds it almost impossible to use my name, even though he's now a seasoned police officer.
My parents in law are known by their Christian names not only by me, but, at their request, also by their grandchildren. Even in discussions about them with my husband he usually refers to them by name rather than as mum and dad.