This is only tough because of the kids. I agree with Leed but think the ex is probably crazy enough to threaten his custody if she doesn't get what she wants. So, while he should stand up to her, he may not want to deal with her jealousy and how that could affect the children. That, however, is his problem. You may have to act like the friendship is the same initially, but maybe in doing so, you can get the normalcy back. I say to just call him like before...talk about work problems or other topics you usually discuss...it may be awkward at first but it will probably bounce back at some point. Let him deal with the ex.

As for the kids, if your friendship/relationship is confusing even to you and him, it is probably more so for them. While he should, of course, be able to have female friends, it sounds as if this friendship is a bit on the border. Until you two work that out, keeping the kids out of it may not be a bad idea. I had something similar happen...a friendship with a guy that had a daughter (I actually bonded more with her than with him since I have no kids of my own)...at some point I ran into his ex, and we exchanged phone numbers b/c she said her daughter talked about me all the time. In this case it was pretty normal stuff...she would send me a quick text to thank me for a b-day gift I'd given her daughter and not much more than that. But, at one point, she did ask me if he and I were involved. I said that we'd gone out once without the daughter around but that it wasn't a romantic relationship. She was cool with it and that was pretty much it....but she did want to know the nature of the relationship and, given that I was around her daughter quite a bit, I found it to be her right to know.