Boundaries and Parenting

I have a very difficult time setting and holding boundaries with my kids(and adults). They are excellent kids (well-behaved out of the house, care about other people, good students etc.) however when they are at my house (I’m divorced) they fight like the dickens over the tv etc. They do not do this at their father’s – who was already dating/living with someone before the ink was dry on our divorce papers. He was very harsh with them that they behave in front of their Stepmom who is very strict. At any rate I don’t know if they feel they can just let their hair down here and are trying to process stuff here but recently the fighting scared me as they will physically hit one another (my daughter is 10 and my son is almost 9). My daughter is in karate and solid and my son is skinny and scrappy – and for some reason finds it necessary to provoke her unnecessarily. This past week my ex came over and said that the kids lied to him about brushing their teeth while they were at my house (they didn’t do it 2x every day) and said they were going to lose their activities, and all screens – which includes TV for two weeks. I got him down to one week and keeping activities (karate & soccer) – this conversation happened before they got off the bus. They were pretty good about it here and didn’t use anything but they were sooooo mad at him (and me initially as they thought I ambushed them) and I think that’s why I’m so afraid of punishing them because I can’t stand having people be mad at me – especially the ones I love the most. Generally I think I have done what you said in your podcast about not properly setting the boundary so I get frustrated until finally I will blow my top and then eventualllly they will come and apologize to me – much harder to get them to apologize to each other but they will (under protest). I would like to start setting more boundaries for all of our sanity but the fear of them not liking/loving/trusting me is so strong. I have a close relationship with each of them and I’m afraid I’m going to jeopardize that. I realize this probably sounds really mamby-pamby but drop me some words of wisdom if you can – thanks!