Aitch you would be surprised at what is a dealbreaker for some people.
Just as you are getting all inflammed about being called Mum (for which I sympathise it is grim) there are others who get inflammed about being called Mrs Childs Surname when that isn't their name.
There are others who get upset when you ask if they the childs Mother, and even more commonly, men who get really twitched when you try to establish if they are actually the childs Father or not. Even more difficult to do is to establish if they have parental responsibility (for unmarried couples with children born pre 2003 this has to be formally applied for). You have to have parental responsiblity to legally consent for children to have operations etc. This is also a bit of problem when granny brings kids to appointments (and I wouldn't ever call her Granny to her face) or when children are looked after in informal kinship agreements by other relatives. I always feel that as HCP I get targeted with a lot of aggro for attempting to deal with these issues but they are important. If I were to anaesthetise a young child for an elective proceedure and invalid consent had been taken (ie not from someone with Parental Responsibility/ a guardianship order)I would be laying myself open for litigation.
Yes, parents names are often recorded in the nursing notes, but it is 1) often buried amidst other information 2) those notes may not be available 3)it might not be there - the nurses on our day case unit have got into the really annoying habit of writing contact numbers down as "mum' 0708098089 and "dad"kjdfjdljf GRRR!
The other annoying thing that none of you have mentioned is when people insist on calling a child "it" or "he/ she (the wrong one)". This drives me to distraction and is one that lots of junior (well, more junior than me) doctors seem to do.
WRT notes/ letters writing d/w Mum is just gramatically incorrect. You should write d/w Mother (and sometimes I am guilty of that because I don't know their name) because legally it is important to document all discussions.
There is also a world of difference between a HCP that you have only met once briefly calling you Mum and members of a team (eg oncology ward nurses) calling you Mum ???? or have i misunderstood? The first strikes me as rude and annoying but copable with. The last strikes me as the kind of thing that would have you swinging for them. I have to see even as a mere anaesthetist (I meet familes for about 15 mins pre op, for around 10mins in the anaesthetic room and then for about 5 mins afterwards on the ward) I do tend to manage to pick up the names of our frequent flyers' parents(usually orthopedic/ oncology kids).
God this is long. Sorry.
In summary, I think you need to tell people what you want to be called, not be pissed off when they question your relationship and then you can be justifiably annoyed if they ignore you.