I share your pain. ALS is crap (I want to use a different word).
Our kids are now 11 and 9 and my wife is now very progressed.
That said, you know your kids, their likes/dislikes, whether their big communicators or shy/reserved.
I can only tell you what we did, not what you should do.
We started with investigating info on the web, getting pamplets and things from our MND association, we also considered getting a physcologist involved.
The kids can see whats going on, once we started with nursing care coming in to the home we recognised that we couldn't hold off being full and frank with them.
We set a date, on a friday and kept the kids home from school so that we all had time to deal with any consequences.
To be brief, we basically asked them what they thought was happening, what they wanted to know.
We then explained MND in simple terms and that mum would not be getting better.
We answered (and continue to answer) their questions honestly (but without too much gory detail).
We re-iterated that although mums muscles are weak, it doesn't mean her mind is.
We focused upon the positive, the things mum can do, the fun stuff, the love stuff.
We all had a good cry and then we had some fun.
We continue to focus upon the things we can do, and don't worry about what we can't.
I wish you all the best.