I was a nanny for an Australian family and was quite surprised to hear the kids call their mum 'mum' - but then they were emotional or unwell or just lovelydovey, they could call their mum by her first name. They would call mum her name in front of me and family member and close friends, but if anyone else was around, it was always 'mum' or 'ma'. It works very well, like there are two levels of 'mum' and I kind of wonder why we aren't all brought up like this.