what does baby call you?

my 4yr fs calls me mom and mommy or my first name. He started slowly and worked his way up to all the time. I let it be his choice what he calls me. His 1.5 yr old brother just started calling me mama or mommy as well. I have always refer to myself by name and not mom to them, however if we are out and someone ask if i am there Mom I do say yes. Its easier that way. I asked my 4yr old if that what he wanted me to do and he did. The parents do not like it and have told my 4 yr old not to do it. The boys have been with me over a year. The thearpist and case workers are fine with them calling me mom. So far the parents are not aware that the 1.5 yr old calls me mom yet. Prior to becoming foster parents, we talked with a friend that grew up in foster care for some advice. He said he hated having to call someone mom and dad. That why I try and let it be a choice for a child. Which is not always easy with a baby.