Ugh. Sorry to hear that you’ll have to deal with this. We’re in a similar boat except the STBX has always had 1/2 custody. Mostly we try to take the high road, although she has forced many issues due to her negative talk and erratic actions. (99% sure she has borderline personality disorder…so we try to see it as an illness.)
With the older children we’ve had to deal with financial issues as she accuses him of hiding assets and money, abandonning his family, and spending lavish amounts. They’ve lashed out at us in return. Our way of dealing with this is to open up our finances completely, showing what money comes in and what gets paid out, including check stubs, bank statements, and copies of bills. It takes patience, and a willingness to practically bite through one’s tongue to not disparage the other parent. Since we are willing to open our lives up to scrutiny and she is not, they are able to see for themselves what is going on.
The younger kids, of course, wouldn’t understand this so we try to explain what we can, as neutrally as we can, in a manner that they can understand without badmouthing. We try to keep a calm, peaceful environment when they are around, show them our happiness, and not talk badly about anyone. As a result, they get to see calm at our house and chaos and anger at hers.
We figure that they all will eventually work it out for themselves, and so far we’ve not been disappointed. There will always be ups and downs, and sometimes the kids will try to use stuff that she’s said to pressure us to get their own way, but we are aware that that is going on. Sometimes it’s hard, but sometimes we’re rewarded with a “Yeah, we understand why mom & dad aren’t together anymore. It makes sense.”