I agree BO the children need to know, maybe you can help her with that.
How old are they? If as it seems she hasn't got long, what will be the arrangements for them. Have they family other than the father who will be about to support them?
Let the school know once the children are aware of the situation, so they can keep an eye out for them being upset/slipping behind, and can provvide support.
Check she has done practical things like make a will.
Ring the Macmillan nurses, and remind them she is ill and in need of support. See if they will fund a weekend away for her and the children or some other suitable treat.
You could help her make a life story book for the children, to remember her when she has gone (not that they'll forget). Or she could do it with the children. Theres a few ways of doing them - if you want some ideas, rather than just the standard photo album, give me a shout.

Tough times ahead BO, when you sort out the internet for her make sure you direct here so we can give her some support as well.

Keep us updated, you know where I am if you need a shoulder to cry on.