Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...
I think the best you can do is be honest with them without adding additional commentary. For example, you mentioned that you think they might know what's going on. If they ask you, it's best not to perpetuate the betrayal by lying to them. You don't have to add any unnecessary adjectives (scumbag or whore spring to mind) but if they come right out and ask you if their dad is having an affair, I think they will be more hurt if you try to protect him. Just convey the message in a business like manner without insulting him and don't give them more information than they ask for.
Sending support
The "fix" wont happen over night. Once things have been discovered, each day is another step forward towards healing, but where you will end up is still anyone's guess. Be there for your kids and they will get through this. Mine were 17 and 19 and knew everything. It was pretty obvious and kids are really observant any way.
Temporary.... permanent......, it will all work itself out over time so dont feel like you have to have all the answers today.
Ponder518, I agree your son is pretty young to know the details, but if you do get divorced, then it is probably appropriate to tell him. A counselor advised me not to tell the reasons unless we actually divorced, because if we stay together, they both have a skewed idea that its somehow alright, or will hate their father.
Just some food for thought.