Whether it's applicable to your question or not, but a "house mother" here in the U.S. is/was a mature lady hired by a sorority house (associated with a university) to run the place. Originally, several years ago, they were there to oversee the morals of the young ladies residing within. It was not unheard of to have the same arrangement for some fraternities as well...
I used to be a House Mother - perhaps house mum is just the more relaxed version of the same thing!
I looked after boarding children at a special school in the evenings and at the weekends, sorted out their clothes, played with them, helped them write letters, generally kept an eye on them while they were not at lessons.