Here in Hawai'i just about every woman is "auntie" and every man "uncle". So, usually I start out with that. I have a bio son who (of course!) calls me "mom", so usually my younger ones end up calling me "mom". I feel (and have been backed up by SWs and therapist) it is more detrimental to the kiddo to say "no! I am not mom, you have to call me "X"". I have been told that it is very common for foster kids to call FPs mom and dad. Of course this can REALLY upset bio parents sometimes... but that is a whole other story.
My older kids who I am in the process of adopting were 4 and 9 when placed and are now 6 & 12. My son who is the older one, asked if he could call me "mom" pretty early on -I think he was more aware of being "different" at school, with friends, etc., and really wanted that family and stability. My daughter didn't see the stigma as much as her brother and called me "auntie" up until the last 6 months or so. Her brother would often try to get her to call me "mom" and I would stop him and tell him whatever she was comfortable with was totally fine with me.
My "baby" was placed with me at 4 months and is now 2.5 y.o. she has been calling me "mom", "mommy", etc., all along. She has had visits with her bio parents all along and we are almost at complete reunification and she will call both of us "mom". This really upsets her bio mom (which I understand) and when the bio parents have her for their visits I know they are drumming into her "she is NOT mom, she is Auntie J", which we have tried to explain to them is the wrong thing to do. Kids have a great capacity for love and at that age it simply is not a big deal to have 2 moms
It is sad to see her come home and when we are saying goodbye to the bio parents she may call me "auntie J" but as soon as they are gone it is "Hi mommy!"