Advice on ex-wife/kids?

I'm in Missouri and just finalized divorce last October, we have 3 kids and have joint custody. Kids ages are 14,13 and 9. The problem is their mother doesn't seem to take any interest in them or their lives. When they are over there, my ex just zones out to movies and never engages in any activities with them unless they beg her to. The only conversations she has with them are usually relating to chores and/or yelling due to not being done. I have talked to all my children and have come to this conclusion....the thing that stresses them out the most in their lives is being over there. They never want to leave when she comes to pick them up. There have been three separate incidents where "mild, but not dangerous" violence has occurred (hair pulling, throwing items, smack in face). All being received from my ex...by my 13 yr old daughter. I have had to go over there and pick up one of the kids on numerous occasions because of agruments. I had to get my kids an iPhone to connect with me because her home phone/internet was turned off due to non-payment. (Been about 10 months since those were working) Here electric has been turned off 3 times in the last 6 months. (and yes I am current on child support) My kids do fine at my home and would prefer to live with me...the older two should be able to choose, but from what I read they have to be 18. My ex has Adult ADD/Bipolar and can barely take care of herself let alone her kids. The problem is that she is not a "bad" mother, per se. She isn't on drugs or having men over all the time, so I can't call DSS or anything to have them removed. She is doing the bare minimum required of a parent and I am clueless how to rescue these kids. Her kids don't even "like" her and not because of me, her actions have dictated how they perceive her. (errant thoughts, irrationality, empty promises)

I have decided if one more act of violence occurs, I am going to take them with me and try to find a solution from there. We only live 15 minutes apart, so I won't be taking them far. I want their mother in their lives, but not if she is doing more harm than good.