[monster]

Do they know what their last name is?

how common is it in Aus for kids to have different last names to their female guardian? What does it imply if they do?

Huge differences exist US/UK. In US parents are most likely to be married, but it's far from given that they will have the same last name. kids will usually have father's or hyphenated last name (surname).

In UK, way more common for parents not to be married, but also more common for them to use the same name despite this. Kids having different last name from mother is more unusual, if mother is single, kids will have her name more often than not. Kids with different last name to mothers are "noticed".

of course these things are changing all the time and I've been an expat for a while, but my sis is a mum in the UK and this is what I hear from her too.