How do you talk about death??

My SO has three kids from a previous relationship. They are 10, 8 and 5.
Their grandmother was diagnosed with cancer about a month or so ago and just let herself go. Sunday, they were giving her three months, Tuesday it was a month...and tonight their mother called SO saying she was heading to the hospital because she might not make it trough the night. His kids are heartbroken and are crying a lot( he talked to them on the phone and I could hear how upset they were from the other side of the room.)

The thing is, their mother refused to tell them her mother was ill...and even when she got the three month verdict she still couldn't bring herself to tell the kids their grandmother was dying...so they learned it today and had not time to get used to the idea.

I have never suffered the loss of a family memeber so I am at a loss of what to do about it. We have them next weekend, so I suppose we should prepare to go to funerals...Ugh I just...

Any advice on how to help them trough their grief would be very appreciated.