OP I don't think you are being unreasonable, but I think your... "friend" is. You said his children consider you just another friend of his, so I assume you have some sort of friends with benefits situation, otherwise he would have (by this stage) surely have used the word girlfriend to them or front of them, in regards to yourself. He's lying to them (and why if they like you so much and you have been together for quite a while?) if you are considered to be a girlfriend. The relationship between your parents partner and yourself is different to that between yourself and other friends of your parents (saying this from experience, my parents separated when I was about 9 and sis was 5). Have your dc's met each other yet?
All he needs to do is send one quick text "Just wanted to let you know, I'm seeing someone, dc's have met her and enjoy her company, if you wish to discuss this, you may do so when you drop x to me a y time". Why is that so hard? For all he knows she may delete the text and ignore the content. In which case he tried to be the bigger person.
I hope everything works out regarding this, he does sound like a nice bloke. You just sound very defensive atm but it's great you are making an effort with his kids.
If your own good friend, who has always supported you in your decisions, has felt the need to comment on this, that should say more to you than anything written here. She knows you (and him?). It doesn't sound as if she was judging you, but him. Maybe it has raised a bit of a red flag to her, and as your friend she felt it best to say something to you about it.
Once he has informed the ex w, he's done his bit (I mentioned "if you wish to discus this" in text as she may have some genuine/non nasty questions). If she tries to start a fight, all he has to do is hold his head high and walk away from her/not engage.