I agree that just being natural and normal around them, and following their lead when it comes to talking about it is the best way. Quite honestly, I don't think you could do anything to make things worse, unless you are spectacularly untactful and come out with comments like 'Oh, I know how awful it is, my dog/cat/distant relative died of cancer...'. They will probably be so preoccupied with what is going on that they won't pay very much attention to anyone else.
If you want to be of practical help, either now or when he dies, don't say 'let me know if there is anything I can do to help'. Everyone says that, very few of them really mean it, and when you are dealing with the death of someone close to you, it is hard to actually pick up the phone and ask for help.
It would be better to offer to do something concrete - if your children get on with them, have the children round to play to give the mum a break or while she's visiting the dad. Or offer to do school runs, or some shopping or laundry, or make some meals etc.
The best resource for dealing with children and bereavement is Winston's Wish, which may be useful in due course.