I have never heard of an objection to being named in a Power of Attorney because you're on disability. As long as your disability doesn't affect your mental capacity, I don't see how you being disabled matters.
I don't understand why your landlord has any obligations directly to you. Unless you have a court order requiring your landlord to do something, it probably isn't his responsibility to ensure that this person not bother you.
Were your children in the car when she was arrested? Has she been found guilty of the OUI?
You certainly can file a motion or have your lawyer file a motion asking that she not drive your children. I don't believe the chances of success are that good unless there are aggravating circumstances about your ex-husband's wife's arrest or her driving record. Good luck!