I'm so sorry you're going through this and by the sounds of it quite by yourself. It's such a hard place to be in, the anger, the grief, the tiredness, the utter sadness of it all. I'm so so sorry love, sending you tons of hugs if you want them, and strength and a peaceful cup of tea so you can catch a breath.
Can you find it within yourself to carve out some time for just you, even if it is just to sit and grieve? Even if your partner doesn't "get" mental health I think he needs to step up here if he isn't already and make that possible for you. Which means taking care of the small things that pile up, like the chores and the shopping and the homework and so on, but also possibly managing your interactions with your mum. Is she lucid enough to put on a mask towards outsiders? I know it's not her fault she's so nasty towards you but some people in her situation tend to mask towards less familiar people still, the ingrained habit of not being too open with people who aren't immediate family. Maybe he can answer the phone instead of you for a bit, because you're "busy". Or maybe he will finally understand more of what's happening instead, which would also not be a bad outcome.
There is always the option of not answering the phone but I guess that is a hard choice to make because we do worry for them and it's easier to be hurt than hurt them, because they don't understand.
Do you maybe have the option to do some grief counseling or talk to a therapist? Or maybe there are support groups around you where you can speak to people who understand and have gone through this too.
About your kids - I'd say be as honest as you can with them (in an age appropriate way of course). Mental health issues are an illness and the more they understand that, the easier it will be for them. Gran's very very ill and it makes her nasty and unkind. Doesn't mean they can't love her, because if they had a good relationship with her before they probably do; it just means that they need to love themselves more and stay away for their own protection. And assure them that it's not on them to manage this, or their feelings about this, by themselves. Have an open door policy when it comes to questions about her, and do share your feelings with them, let them know they're not alone with missing her or hating her sometimes for her behaviour or any of those big big emotions kids feel. Make space for their feelings if you can.
And finally, how much of your mum's care rests on you shoulders? Is there anything that can be done to get more professional help to take some of that load off your shoulders? Any meds she could take that might help her stay calm? I know Lithium has fallen out of favour a bit but it sometimes can be very helpful when people are so deeply stuck in their psychosis. Additionally, has she been evaluated for dementia?