I'm so sorry to hear of your Mum's illness. My own Mum died 18months ago.
We have lost all 4 grandparents in the past 12 years, first one when my kids were little, like yours and then all way up through teens. Mine both died of cancer
I didn't go into any detail when they were little about their grandfather's iminent death. They won't grasp it. I have always answered their questions truthfully, as they got older. I have judged what to tell them based on what they seem to want to know. Macmillan gave me a brochure about talking to children about cancer. Available online. They had useful tips. The beauty of children is that they are resilient, and to an extent selfish. They have to be to survive life! They will be more upset by seeing your grief, which is also fine. Sadly it's part of life
Look after yourself. The children will be fine. I wish you every strength.

Dear mummy 2018, I’m so terribly sorry. I lost my mother when my children were 4.5 and 2. It was also cancer so we had a few weeks to prepare. As painful as that time was, we were grateful for having the opportunity to properly prepare the children.
Another mum in reception had just lost her father and had told us all about the book “Badger’s Parting Gifts”. We read it every single night. It’s a beautiful book with a beautiful message. Elderly Badger, loved by all, dies, all his friends hibernate during winter, staying indoors and feeling sad but when spring comes they all venture outside, laugh, dance and play and they all express gratitude when they discuss the things he’s left behind for each of them. It teaches kids that mourning and being intensely sad is OK and completely temporary.
When the day came and we had to tell our reception age boy that Grandma had gone to heaven, he stopped in his tracks, wide-eyed and said “Just like Badger” and then at various points throughout, he’d comfort adults and say ‘I know you’re sad now but one day soon, you’ll be less sad and just grateful that she was our grandma’

It’s on Amazon. I wish you all the very best.

Dear mummy2018,

I'm so sorry to hear you're facing this hard, hard time. Good on you for being so open and thoughtful about how to support your young children at such a tough time for you all.

As TJ74 has suggested, Winston's Wish and also Child Bereavement UK and Cruse have some excellent resources.

We at Poppy's have put together some helpful and accessible resources which I'm trying to include here but Nappy Valley doesn't like me including links. I'd love to help though. Call me or us on 020 3589 4726 and I can email them over if that feels helpful. Or you can google Poppy's and Talking Death and How to Talk with Children about Death and you'll find them all.

And it's hard, but remember to tend to yourself too. There's no doubt you'll do a great job supporting the kids, but you are going through this too and you need tender, loving care from all those around you.

All my very best wishes,

Poppy

My mother died of cancer when my daughter was two and a half. I didn’t tell her how ill she was before she died. Once she had died I told her obviously but she didn’t understand it at all. She asked me lots of questions about where she had gone but couldn’t grasp the concept of death.

If you tell your children while your mum is still alive will they ask her questions about it and could she find this upsetting?

You have my deepest sympathy. It’s never an easy time to lose a parent but when your children are so young (my son was a baby) it’s incredibly hard. I was so depressed and upset at a time when I should have been enjoying my kids. Look after yourself and get some counselling if necessary. I didn’t and it took me a long time to get over it.

Dear mummy 2018,
I am so sorry to hear about your mother's diagnosis.  I do have some personal experience as my father-in-law received a similar diagnosis when my children were around the ages that your children are.  I contacted the charity Winston's Wish (https://www.winstonswish.org/) and spoke to someone about how to tell my children that their grandfather was terminally ill and how to best support them, and the person that I spoke to was incredibly helpful.  They gave some very good advice and also sent me some resources.  I would recommend contacting them.  I hope that you have someone to support you too.  

Hello mummy2018
I'm very sorry to hear about your mother's diagnosis and the heartache you are going through at this time. 
I recently joined a webinar called 'How to talk to children about death' that was led by wonderful Poppy's Funerals in Wandsworth https://www.poppysfunerals.co.uk/
I know that they recorded it and I'm certain they will send it to you if you contact them. They are the most wonderful team of ethical funeral directors and will help you in every way as you prepare to talk to your children (irrespective of your choice of funeral director).
I was lucky enough to meet Poppy at a local community event where she was talking about her mission to be a resource and support for people experiencing death. There are many features on the website to share information and help you navigate your way forward for yourself and your children. 
I unexpectedly had to arrange a funeral after meeting Poppy and I decided to try out her service - it was absolutely flawless and delivered with such compassion as well as transparency (how my deceased relative would be cared for / funeral options & costs etc) 
With my warm wishes and best thoughts,
sunnyjan
 

My mother has very recently been diagnosed with a terminal cancer. She has always been an amazing but since I had children she has has been the most loving grandmother. She looks after the children once a week. I am really struggling with the imminent loss of my mother but just as upsetting for me is the children (they are two and four) losing their grandmother, I just don't know how I am going to tell them or how they will deal with such a huge loss. Should I be preparing them or do I wait until the unthinkable has happened.

Would anyone be able to offer any advice if they have had to deal with anything similar. Sorry for such a sad post.