is he depressed!!! u should not tolerate him hitting your children or grabbing them by the arm so hard, they are such small delicate little darlings and they would be in pain, so what if they are naughty, put them on the naughty step/corner, take away a favourite toy for half a day until they behave, but hitting them is against the law, and even if it wasnt against the law, it should never happen, u as a parent should protect your children and not let anyone, even the dad, hit them. does he see fit to hit the kids, was he hit as a child, he seriously needs help, and of course the kids will love him, thats all they know, coz he his their father.but bruises could lead to you losing your kids! and you should talk to him and tell him if it happens again you will leave him, or better still throw him out, ur kids should come first