Wow, first of all let me say I can not imagine the hurt and anger you must feel right now. Secondly let me say that this is an issue between you and your (soon to be ex) spouse. There is absolutely no reason to tell a 7 or 9 year old the details about what is going on. I can not be any clearer on this! Divorce is hard enough of children as it is, and clearly you are headed that way (with every right), but they will have enough to deal with without adding the reasons why to the situation.

The only thing the children need to know about the divorce is that they are NOT the cause. They do not need to know what the cause is right now and if you start out telling them you open a door to using them as a confidant/friend/sympathetic ear for you through this VERY difficult time.

That is a lot of pressure to put on one so young. They did nothing wrong in this situation. They love you and your spouse and while what they did was clearly not right, they still deserve the love of their children. If you take this to the level where you bring the children in, there is no going back. They will figure it out later and when they are adults then you can sit with them and have a drink and tell them how you tried to protect them. But they are not going to be able to support you either emotionally or physically right now.

My recommendation would be to get involved with a church or other religious group who can give you the adult support you need in order to deal with the infidelity. Let your children be children for as long as possible, that is your responsibility to them, just because your spouse broke their trust with you, does not mean you have the right to break trust with your own children.