Why would a teacher / staff member discussing a child with that child's own parent be a sackable offence?
Whether sackable would depend on the content.
It is however very unprofessional.
Staff choosing to go off record and chat to parents about school things in a personal capacity isn't appropriate.
E.g. They may have a snapshot of a moment, but then go gossiping with the child's parent instead of behaving professionally. The parent decides that their mate's version is a full version and turns up at school kicking off.
E.g. It undermines professional trust between colleagues because what is said in the staffroom stays there. I'm not talking bitching here, I'm talking actual concerns. (E.g staff discuss whether DC needs additional literacy, but friend passes this on / they're not happy with DC behaviour and the way a group of friends has acted at playtime so they're going to lose time, friend tells parent who then comes in making a well timed claim that their child is caught up with some nasty children)
E.g. Some information in school is need to know so some staff will know more about a child than others and some might know information about a child that their parent doesn't. That's not appropriate to share.
Eg. Confidentiality issues for the child and them having a reasonable amount of privacy (say a child has requested access to a mentor/counsellor in school but this isn't shared with parents). Someone decides to tell the parent that they saw their child upset after counselling / saw the child upset today and wanted to let home know (without knowing what was going on). Linked to that, the child may be being a typical teen but in a way religiously conservative parents would have an issue with but the staff member who is a friend of the parent acts as spy passing information back.
E.g. Friend likes the student and so always tells their mate (the parent) how great their DC is in school, how they're amazing etc... The problem is that when a teacher has to call home about an issue, the parent has already been told that their DC is a delight in school and everyone loves them, they're really popular etc (can be an issue if they're not actually popular but a bit of a madam/intimidating bully because by then home have been told how great and lovely and charismatic their golden child is).
Eg. Friend who works in school tells parent of child part of a story, parent passes half the information on the sly to someone else and then Chinese whispers happens about a child, family or teacher.
I used to work at a school where a lot of the support staff lived in catchment and it was a very, very small world (most of their kids had or did attend the school). One of the interview questions was how you'd respond to the question "heya, how's Tim getting on at the moment? Please tell me he's settling down" in the supermarket. The line was clear: we didn't discuss it and told them to contact school.