My Mother has POA and her husband has Alzheimer's and was just put in a home.Can his kids take POA away?

And get letters from two or three doctors that specialize in geriatric medicine and/or neurology stating that he is incapacitated and unable to make those sorts of decisions. Then if kids somehow get it changed then you have evidence that he was not competent at the time.

Do they have grounds that your mother has abused her authority under the POA, which I'm assuming is a legal/financial one? They could try to challenge her and ask for a court appointed guardian, but they have to have legitimate, verifiable accusations.

Why would they try to challenge doctors, unless it would be to refute the Alzheimer's Dx so that your father could legally make changes to his DPOA himself.

If you expect this to happen, it would be advisable to see an elder law attorney. If you can't afford one, do some research on your local state bar association and look for pro bono legal groups or attorneys.