I think that you or DH need to take her aside and say, "It isn't working out for your girls to be here while you clean. I'm sorry but you will have to find another arrangement for them the day you clean."
If she gets upset about it or asks why, tell her simply that you have found evidence of them being into your children's toys and writing on the wall/breaking things and can no longer ignore it. You can also tell her that "they do not treat the dogs appropriately and I cannot risk either harm to the dogs or harm to your children if the dogs get fed up." You also have the witness of your aunt. Do not let her wiggle into more chances. You've taken a long time to come to this point, and should stand firm. You are not firing her, just telling her that the special exception is no longer an option.Truthfully, your dogs could bite the kids in fear/retaliation and then what? Will she admit they were being terrorized? Will you be reported when she takes the kid in for a dog bite? Will the child have scars and medical bills? Etc.
It is hard working with family, but at the end of the day, as much as you want to help her out, it is not helping YOU.
Her options after that are to quit the job or find a friend or sitter for the kids for the few hours she is there. I freelance and anytime it is not appropriate to take DD with me, I find a friend to babysit for me, or I couldn't do the job/meeting. Sometimes it is simply unprofessional and not an option. If she quits, then that is her choice, and you can find someone else.