Englishman – If you live in the US – go back to the UK. Here in the US, not a chance in a million.
The only way possible is that, besides costing you an arm and a leg to do it, you need a good lawyer and a good private eye. Even if she’s nuts, you won’t win. Because your shrink will say she is and her’s will say she isn’t. And vice versa -her shrink will say you’re nuts and your shrink will say you’re not. So the court gives her the kids.
So how is it ever possible? Back to what I was saying – you need the private eye to spy on her and document, like with videos, her behavior. Very few actually will harm their kids -so this documentation you won’t have. The same with any illegal activity. Very few actually break the law. But a lot, not all, will be negligent, ie. leaving small kids alone at nite to go out. This quite often occurs and if you document this, without her knowledge, it’s quite possible you can win custody.
The only other time is when the kids prefer the father over the mother. This is very rare, but it does happen – once in a blue moon.