My husband's parents divorced when he was in 5th grade (I think that was the age). I don't know if your characters are Christian but I can tell you custody cases can get pretty nasty, regardless of how much both parents love their child/ren. In my hubby's case, they battled for a year, and all the anger his mother felt at his father for everything leading up to the divorce was manifested in that custody proceeding. Hubby & his brother were caught in the middle with their parents unintentionally drawing them into the fight; Hubby feeling he needed to be there for his mom while his younger brother drew closer to his dad. But she let her desire to get back at him override anything else. She won custody and his brother ran away repeatedly, wanting to be with his father. Eventually his father was forced to actually leave the state, just to prevent his son from running off all the time. And as soon as he turned 15, the courts let him decide who he wanted to be with and he was off to live with his dad immediately.
I'm not sure what the deciding factor was for why she got custody, though I'm guessing his PTSD and disabilities after Vietnam (making him unable to work) didn't help his case.
My sister also had a long, bitter case but hers wasn't about custody so much as visitation. Her ex has massive psychological issues (hallucinations among other things) and he wasn't allowed around the children unsupervised. Oh, and two women I know from Bible Study have grandchildren in the midst of custody cases. In one, the mother had a terminal cancer and the father was given custody after she passed, even though he had a history of violence and she had wanted custody to go to her parents. They are still fighting it. In the other, similar situation only the mother is fine. He was granted joint custody despite documented massive psychological abuse and temper issues. The initial ruling, the judge wanted to think about it for 2 weeks and the dad blew up, nearly got jailed for contempt, and yet he still gets them every two weeks.
It's a sticky, tricky issue and I'm sure different states have different rules that factor in as well. The above cases I've mentioned took place in Wyoming, Florida and Colorado.
Here's an interesting article about the process:
http://www.wwdlaw.com/article/123/what- ... t-child-cu