100% totally had this in my family. I really think that it is fine to just let the parents be parents, and if they have the desire to change the baby all the time then it should just be understood as what is normal in their relationship with their child.
I have four daughters and they all had kids at the same time. I was overwhelmed with all the different rules for each family. I got in trouble a few times for various honest grandparent mistakes for which I was called out for.
I was never good at settings boundaries with my own mother, so when my kids began setting boundaries, I had to get up to speed.
Setting boundaries and actually saying the words is helpful. I have also set some boundaries about what I am willing to do or not do. My daughters were a little taken aback when I had a few boundaries about my time, but since they had already set boundaries about things it was easier for me to do the same without animosity.
There are so many parenting styles maybe your daughter or son have a particular style. It is always helpful to understand their style. I actually had to read up on Gorilla Parenting,
Original Montessori, Cry is Out, Home Schooling, ETC.
Good Luck, just read the room and go with their flow.