This is the text:
Benny laughed, "She can't buy electricity in packets like sweets! She'll look foolish." Mum said, "You mustn't say that, Benny!"
My confusion is: Is it proper for mum to say with "mustn't" to her son at home?
The 'mustn't' part came across as something said by an old-fashioned mother, I agree with Donny, surely. But I did not think anything of it because it may well have been said long ago. Sounds like the mother is reproaching Benny for being rude, generally, teaching him manners.
There are millions of ways to reproach your children. I would probably go with You can't talk like that, Don't be rude, You can't say such things, etc.