similar to you... we are trying the same.... but slightly different
First, I'm sorry that an affair took place. It's always really difficult when honest communication was not explored from the beginning. Trust becomes an issue and it's hard to backtrack on that.
Same thing with us (I would be your wife in this scenario), but my husband and I talked _before_ an affair took place (because he sensed it would eventually).
I have no profound words of wisdom, as we are still new to having an "open" marriage. We have two kids (10 and 8). But I will say that it will take a long time for things to settle into your new "normal" and it will require oodles of open communication and patience and understanding. You will go through the gamut of gut-wrenching emotions. Things will feel pretty good, and then you may feel the rug just got pulled out from underneath you. This is going to take a long time.
Find a poly / open-relationship friendly counselor. That is one thing I would highly recommend. It sounds like your current therapist will just try to steer you both back to conventional monogamy - and it may be all that person knows in the scheme of how they were educated / brought up / taught to believe. And that won't be good for you and your wife since you've decided to embark on an open marriage path.
I think it's great that you both want to raise your kids as one unit (not divorce), and are exploring a way to do that while also trying to fulfill your relationship needs in a respectful way.
We are doing that, too (but unlike you, my husband and I are primary to each other and still enjoy each other emotionally and sexually).
My first reading resource was "The Ethical Slut" - I found it very informative and it gave me hope. I am currently reading "Non-Violent Communication" based on a recommendation from some poly friends (just started, so have not formed an opinion yet). And peruse this forum. You'll find some great insights here with lots of people sharing from their experiences. Good luck! -D