I have been giving this a bit more thought since my last reply. And it has occured to me - if you come across as wise, sane and generally sorted in real life as you do on here (and I bet you do, whatever you think!) I wonder if Mr Tree has you on a bit of a pedestal and just kind of thinks you would be above any sort of jealousy - assumes you must know how great you are...... And so he isn't being as careful of your feelings re. the exW as he should be - but that may just be because it doesn't dawn on him that it would worry you? Does that make sense?
It's difficult, because I totally understand your reluctance to bring it up - and anything that involves someone's kids is always a sensitive one, even when they are adults.
It was my bastard ex Mr Tortoise who did all the family days with his wife and kids - we even all went to France for Xmas once! Weirdly, I wasn't bothered by her presence - I think because I got on with her so well. But I do remember one of his kids (the one who liked me the least - I was close to two of them and less so the other two) used to organise stuff and intentionally exclude me, and when I got upset about this once Tortoise bascially told me I was a needy mess and did one of his regular break ups with me. (He broke up with me on around a monthly basis I would say - twat!)
So yes, I think some kids, even adult ones, will weaponise this kind of thing if they have a reason to dislike a new partner - but Tree's have never met you, he's had other partners, it's hard to see why they would. (Though also not impossible).
I suppose one possibility (and please remember that I know nothing) is that his ex has decided she is still attracted to him and is encouraging the kids somehow? I know I am generalising, but most humans are fundamentally loyal to their mums...... If so, I do think she is put of order. I realise I might be in a hypocritical position saying this (as I have admitted attraction to my ex, and while he is driving most of our meet ups, I am certainly happily making myself available for them). BUT I wouldn't do that if he was seeing an "innocent" woman - anyone, basically, apart from the woman who was instrumental in put split. Girl code iIS mportant!
Sorry, it may not be helpful for me to say this stuff, if you are over thinking anyway. If he still seems as keen on you as he ever has, I would probably try not to worry too much. But if you detect any change, I would say it's completely fair to raise your concerns with him....
But I honestly think my first paragraph is the key - you seem so amazingly wise and cool and strong that he probably thinks you don't even do jealousy! (But we pretty much ALL do jealousy. And rightly so, frankly!)