My kids are a lot older than yours (23 and 18) and they know the whole sad saga. My youngest was talking to a good friend of mine a while back and said that she is glad we have kept her in the loop as she would hate to be wondering what is going on and be too afraid to ask. She knows she is free to come and ask any questions she has.
I discussed this very thing with a therapist shortly after D-Day - I was concerned as my younger daughter had seen me VERY upset (as in on the floor in a foetal position, beyond crying and more or less in shock) The therapist said something that really stuck with me, she said "in families there should be NO secrets, only surprises" She went on to say that the minute you allow secrets in the door you are potentially allowing all sorts of nasties in too, such as kids keeping sexual abuse from their parents, kids keeping drug use from their parents... etc etc. She said surprises are fine (eg "we aren't going to tell mommy about the pretty scarf we bought her for her birthday, its a surprise") but secrets are NOT, never under any circumstances and she said it should work both ways - parents don't keep secrets from the kids and kids don't keep secrets from the parents. Her feeling was that this is what families should be about, there should be that trust that we all will tell each other what is going on in our lives. (Yeah, wish that had been the case with fWH and me!)
She also said that if the kids find out from someone else in a situation like this they are going to feel pretty much the same way a betrayed spouse would feel, in that there were huge things going on in their life and no-one had the decency to tell them.
It's of course a very personal decision, you know your situation, you know your children. Obviously all info needs to be age appropriate, in your case the bare minimum is probably enough.