When I told them she had a son and his age they giggled. They have also not seen me be affectionate towards any woman since their mom left over 2 years ago so I do suppose they will be observing. I think it will be a little weird for me initially as well and I assume the same for the Dr. as her son is in the same boat. It will definitely be a feeling out period for the both of us.
In many ways though my XW has absorbed the burden of most of it since she introduced her BF to them over a year ago. They should be used to it by now since they have spent time with them.