The children likely cannot do anything, they are too small and don't know any better. If you see such things done (severe beatings), you should advise this person to stop and this is unacceptable and great sin. You cannot hit anyone that hard, let alone sinless innocent children- this is just common sense too. If he does not stop seek help from other Muslims and threaten to get the authorities involved. He does not sound like a fit parent, he should not be around any kids if he is constantly doing this. Try to do it in a way where he cannot beat the kids worse or run away with them.
They are his children so be-careful not to overstep your boundaries, he has a right to discipline them, but he should not be this harsh.
Tell the kids to listen to their father!