First congratulate yourself for being willing to take accountability for your responsibility of being a father.
Just know, you have to be upfront and honest about everything and how this lead you to wanting to take accountability.
Also just be VERY aware that her and her family will not just accept you with open arms and act like what you did didn't negatively impact the family. Others had to step up to the plate because you fled from the situation for whatever reasons.
Be honest and upfront with her, be supportive of your son and be one of the best damn fathers out there in the world and compensate for this mess up. We all have or had fears that makes us do something very similar to this, so not a single individual has a place to judge.
Just be aware that this did impact her and her family negatively and your son is the one who was impacted the most by this.

You won't be able to reverse the damage already done, but what you can do is give her an apology, apologize to your son, and finally be there for him. Just because you made this big mistake, doesn't mean you don't have the power to turn this negative situation around for the better later on.