The only thing that's really changed (other than us moving) is their mum now has a DP, we don't really know how long for but it is at least 12 months that the SC have mentioned him. Prior to that they didn't know about him and at first he was introduced to them as her friend. Which I totally understand from her point of view. I was never introduced to SC until after we had been together a year (we didnt live together at first) but they knew about me a couple of months beforehand. Her DP has a young DD, and we know that they used to do things together. However DD has now moved away due to her mum's DP work for the foreseeable, which we know EW is delighted about as she made a flippant comment about this to my DH. We obviously wondered if this has made them a little more insecure as this is mum's first long term partner (we think) since they separated. We know there have been a couple of let's say 'dates ' or 'relationships' says for a few months and were pretty sure the children didn't know about these and as mum's never said anything neither have we. Mum has told us that she has told the children they will always come first and her DP will never live with them. Now that's all well and good but what if that changes? They decide they want to get married or something ? Anyway that's up to mum it was her decision to say that.
EW is a very very controlling, jealous and materialistic person, (and its not a case of just what my DH has told me his family are well aware of how she is and have told me themselves) I had a very long chat with DH BIL, about 2 years into our relationship and I could not believe some of the things I was told - but that's a whole different thread !). Now don't get me wrong DH was not an angel either and there was wrong on both parts, how they lasted 15 years is beyond me. But he's also never told the children anything about the relationship with their mum. He thinks, and I agree they are too young but he is willing to talk to them when they are older.
We have discussed this problem at length, why has it reared its head more problematic now?
We've never thought of it as you suggested that the desire to hurt EXP is so great that it doesn't matter how it's done. I suppose they really are using their children and so focused on their goal they are blinded by what it is doing emotionally and mentally to their children.
I have relatives and friends who have separated with children and not yet have I come across a situation like this. They are all very adult in the way they talk to and behave in front of their children, even where infidelity has been the cause.
Only a couple of days ago DH suggested that maybe it was to try and create a divide so that the SC would not want to come and cut them out of his - and our lives completely, it would look like their decision but because of the manipulaton she was really behind it. And he thinks this even more so now that her DP DD is out the picture (really not sure if there is any contact) she wants to have her DP , kids and play 'happy families'. My initial reaction to DH was don't be so silly, but the more I think about it the more be could be right.... I really hope not.
I Have felt incredibly sorry for my DH reading the texts he gets sent, listening to the things they say to him, sometimes its heart breaking.
I just hope we can get past this, but I do know that if we do anything with out them, it will bring it all up again. And we have agreed that our DC are not missing out because we can't logistically get 6 of us somewhere. SC come with us to everything that is possible and sometimes we dont go to things if we all cant go. Last summer there was a milestone birthday celebration for one if my DH relatives in another part of the country. We can't fit 6 in our car, train was working out incredibly expensive, we would have needed 2 hotel rooms... so we didnt go. Could we have hoped in the car, 1 hotel room, yes but DH didn't want to leave them out of that.
Like you I'm thinking thst as they get older, they're going to be less likely to want to come and will have matured a bit and not be that bothered, and ideally woken up to the manipulation from mum and risen above it ?? Too much for me to wish 
Wow ! That was long - thanks for reading if you got to the end !!