my LO calls my DH by his name, but I am "mummy dwells" - when LO arrived, s/he only had a handful of words. LO's sibling started calling me mum after 2 weeks and refused to call me by my name (despite daily requests) - a symptom of his/her very broken attachment - and so LO followed suit.
I find it hard because you dont want them to feel different at the school gates, or different to their "siblings" (real and/or foster). The older sibling knew and understood I wasnt his/her mum, but needed me to be that for appearances, so s/he did not have to say "I'll have to ask my carer" . "Oh, whats a carer?/why are you in care etc etc."
Our SW seemed not to be bothered.
I also think its important because one of our LOs would become very confused - would say mummy hit me, but meant BM, not me. But because they said it, I would have to report it.
I also know an adoptive parent whose child told the teacher what his dad did to him, and that launched an entire SS investigation ...turns out he was telling the truth, but about his violent birth dad, not his lovely adoptive Dad.
it is safer for them to call you by your name.