I see OP's point. What if he manipulates the children into hating their mother or believing her to be a shrew and a nag? What if at dad's house, there's ice cream for breakfast and no bedtime? What about when they start asking to go be with him instead of her, because he has money and no rules, while their mother is rule-bound and doesn't let them get away with everything?
I've seen this exact scenario play out. When I saw it happen (with a friend), the DCs have ended up borderline sociopathic, call their mother "bitch" and "cunt" all the time, are misogynistic assholes, and think the sun rises and sets on their cunt of a father.
Children aren't stupid, but they're also not always able to see manipulation for what it is, and if they're manipulated as children, they may be too brainwashed to get out from under it as adults. That's why we don't let children sign contracts and why we don't let adults engage in romantic/sexual relationships with children: they're terribly vulnerable and cannot see what's best for them. With hindsight, perhaps OP's children will tell her she should have left their bastard of a father, but if she actually does, it's possible he'll get his claws in so deep that they'll never really come out.
I'm so sorry, OP. You sound stuck between a rock and a hard place. I have no particular advice to offer, just a perspective.