R.--
I'm sorry you're marriage is falling apart, but I'm glad you at least seem to have a level head about you concerning the whole thing. I've been through it, and its not pretty, no matter how amicable it seems to be. Please watch your back and once the papers are filed, trust no one but your close family and friends, and your lawyer (providing you have a good and trustworthy one).
Where the kids are concerned, you really will just have to be as honest with them as their intellegance level will allow. Unless he is abusive or neglectful with them, do not prevent a relationship between them and him. If he chooses to not have a relationship with them, just be there for them when they need you and make sure they know that this is not their fault in any way. If the latter happens, he will dig his own grave, and as sad as it is, the day will come when your children realize exactly what happened. Not for a very long time unfortunately, but it will happen.
As for the financials of it all...you absolutely must separate everything. It is really better to do it before you even file the papers. Once you file those papers, you are putting your life (and your childrens' lives) in the hands of the court. File for child support with your initial divorce complaint. Do not wait on this. Even if you can survive without it, do it anyway. Even if you don't think he'll ever pay, file anyway. Get the order in place. If you receive payments, but don't "need" the money, stick it in a savings account and leave it be. Don't ever get to where you depend on it for income though, because if he stops paying for whatever reason, you'll be in a really tough spot. All of your accounts need to be separate. You can not count on him paying anything monthly to you or anyone else. You don't need to find out after you've been in a car accident that your insurance lapsed because he didn't pay his half, know what I mean. My divorce lasted 2.5 years, so I feel like I have a pretty good understanding of what should and shouldn't be done. Feel free to message me if you have any other quesitons. Good luck to you.