If you have anything to do with children whose parents are divorced, one thing you should never do is talk bad to them about either of their parents or try to set them against one of their parents or try to diminish their respect and affection for one of their parents. Anyone who does this ranks among the douchiest of the douchebags in the universe.
This really puts the kids in an awkward and hurtful situation and will ultimately make them resent you when they figure out what a toxic piece of shit you really are. They probably get enough of this from one or both of their parents without your adding your self centered douchebaggerry to the mix.
Stay out of it. Don't take sides. Put the interests of the children first for once in your life. It's not your divorce. They're not your kids. If you are affiliated with one of the parents, let that parent know that it isn't cool to try to alienate the affections of the children from the other parent. Don't enable or cooperate in such behavior. Even if one of the parents pisses you off, don't take it out on the kids by screwing with their little hearts.
Why am I even writing this? Meddling, child manipulating douchebags will be meddling child manipulating douchebags no matter what.