I second this one. In my safety deposit box, I have some letters prepared for certain events from my grandmother she wrote before she passed. Marriage, First Child, Etc. They're clearly labeled to be opened at certain events. Do the same, key events in their lives. Granted you don't want to go overboard with them (IE every special event), but perhaps for the next few years, you can pre-write Birthday cards through X age for each, Driver Licenses, Graduation, the same list as above.
It'll be a way for her to pass on her knowledge and life experience in a timely manner to them, and be able to still participate in a bit of their raising long after she's gone.
I'll also weigh in on a comment above. Do document, don't over-document. Unless you're going to be editing the footage down to some reasonable quantity, it'll mostly be for naught. Remember, every minute recorded takes a minute to watch. If you have two hundred hours of footage, most aren't going to want to sit through it unless looking for something. Diaries, journals, are excellent in this regard. They can be read much faster than written, can be picked up and put down for most with little need to go back and re-read the last half an hours worth to get back in your mind where you were. That, and if she's going through treatments during these final days, her personal image is going to start to degrade. It's going to be hard to watch it personally, but do you want your children to watch the entire thing? Probably not. Books let you maintain your own image of her, even when she looks her worst. Of course you will want key things recorded and that's OK, but most want to be remembered at or near their best, not arguably their worst.