If you want a proper relationship and to at some point meet his family and children then you really do have to step back now. If it all blows up and you are exposed as the OW the family will never accept you, his children will hate you. They will be unbelievably hurt. They are innocent in this, as his wife, and the children will see their mum hurting. I'm not saying this to be nasty, it's what is likely to happen.
If you stop sleeping with him and step back, it will give him the space to make his decision and to find a way to end it with her decently (as far as it is possible to end decently in this situation) if that's what he decides to do.
Whatever happens, you should be realistic. He's not going to be able to introduce you to family and his children for a long time even if he does leave the family home.
I would be surprised if he moves a long way from his kids though. My son's dad moved 60 miles away back to his home town after we split up. That's too far I think, so 100 miles + is really pushing it. If he loves his kids I can't see it happening. If he does, how will you feel about him going off every weekend? You would not be able to join him. During the week you will both be working most the day, then he will be away at weekends. Very little quality time together.
The reality is that this is really going to be painful, for you as well. No matter how it pans out it will be painful and difficult. Is he worth it? Nobody can answer that question, only you.
I know you're emotionally invested in him but please try to be objective as much as you can, for your own sake, if not for his children's sake. Try and step outside and view the situation as though it is happening to someone else. Consider what life would really be like with him. It's not going to be rosey in this situation