For the benefit of other future in-laws reading this thread I wanted to share my story. My inlaws never addressed the name issue. I never mentioned it because I thought it was their prerogative so I continued to call them Mr. and Mrs. last name. Later, I wondered if they thought the name should be my choice but at this point it went on so long that discussing it felt awkward. Finally, after five years of marriage, we had children and I started calling my inlaws by their Grandparent name.
My inlaws are formal and traditional but we are not talking Downton Abbey There has never been any animosity between us but the whole situation was very awkward for me. Unfortunately, their continued acceptance of the Mr and Mrs moniker made me feel that I was not truly a part of the family.
So, if you want your children’s SO to call you a particular name, please discuss it with them.