I'm with you, I don't think lying to the kids is a good idea. I'm sure they'll see through the "just friends" ruse in a minute. Even if they didn't think it was suspicious that daddy suddenly has this new woman friend they've never heard of, I'm sure H and OW won't be able to refrain from PDAs for a whole day. Plus how on earth could her kids keep their mouths shut all day, even if they were told to? Surely something would slip out about stuff that your ex has been doing with them, or about how much fun they had last time Ex slept over at their house, etc etc.
But you're also right - you can't keep it from happening forever, and actually, there is still a chance that the reality of dealing with all those kids together might cool off the fantasy a bit.
I suggest you explain to Ex what I just said, about how there is no way the kids won't figure it out, and suggest that he sit down with the kids ahead of time, explain that he is in a relationship with a new woman, and that he wants to take them to meet her and her kids. If they freak and give him a hard time about it - fine, he has to face reality. If they accept it, at least they'll go into the situation knowing what's going on so they won't feel even more betrayed when they figure it out.
Tell him to be a man and tell the kids ahead of time - in person.
