Yesterday we had "Thanksgiving" with my step-daughter (4 yrs old) and in-laws. I believe my in-laws go by the names "Pap Paw" and "Grandma". My stepdaughter, who adores my FIL, attempts to call him by his name but it winds up coming out as Papa. It's really cute to see them together but I noticed she has picked up on him calling my MIL by her first name. Every time she referred to my MIL yesterday, she called her by her first name. I cringed and prayed someone would say something but they didn't. When we left, I informed my DH that it bothered me as I felt it was disrespectful. However, I don't know how to explain this to a 4 year old that it is okay to call her stepmother, me, by my first name but not other adults. I'm also unsure other than "I'm the adult and you're the kid" how to explain why it's fair that I get to call them by their first names but she doesn't. My husband agrees that he feels it is disrespectful but he doesn't know how to address it either. Has anyone experienced this before? Any advice?
As a side note: We do not have custody of the child. We take her about 3-4 times a year to DH family functions and all the other visitations are either just with DH and I or just with the in-laws. Not many opportunities for behavior modification. Should I just leave it alone and hope she calls her by "grandma" or "grammy" or something when she sees our baby starts doing it?