A thought: Isn't it possible that she "always" wants her husband to babysit your kids, as well as her own, in hopes that if your kids are present, he might treat his own a little better? Just something that occurred to me. She may not even realize that that is her motivation but it might be. And it would speak volumes if she needs other kids to be buffers between her children and their father.
Always, always go with your gut. Your gut is telling you not to leave your children with this man, ever, period. Use what Queenofthecastle posted; it's direct. And if she is hurt enough by that to dump you as a friend-- how very sad for her, if she cannot hear the truths she has, herself, spoken repeated back to her by a good friend.
Hire a sitter and say that your sitter will take all the kids, yours and hers, so you and she can go out. If your parents are up to it, have them take all the kids. But if the only way this mom will go out is if her soon-to-be-ex babysits -- you need to tell her that you are not comfortable with that. And then you need to stick to it and not cave if she gets upset or angry or weepy or even if she sobs that she really really needs this "girls' time" away from hubby. If she needs it enough she will accept that your kids don't stay with her husband. Put the kids ahead of the friendship.