***NOT AN ATTORNEY***
It's hard to answer without knowing the complete particulars of both your and your ex-wife's situation. For the most part, getting into that much detail in a case would require that you hire a lawyer to focus on your case. (This is why nothing on this has been answered yet.)
What I can tell you is that if you can provide proof that the kids are with you currently sleeping at your house 5 times a week, or if they are spending the majority of their waking hours at your house, you have the better case. Let me say this: "It is all about what you can prove." Document everything you can. If you can get it in an email that the reason she wants the kids is because she doesn't want to pay for child support, good. If you can get it in an email that strange men stay over on a semi regular basis, good. If you can prove that you are the main source of care, transport, feeding, clothing, tending, taking to the doctors, attending PTA meetings, then cool. You've got a much better case than "I'm trying to switch to 1st shift so I can see my kids more."
Courts deal with this kind of BS all the time and commonly can see through it. All you have to do is have the stronger, more tangible proof. Stay calm and try to make sure that the other parent gets plenty of time with the children (you don't want to come off as vindictively withholding the children from their mother), just show that she's not the most stable person to be the main caregiver at this point in time.
If you have to draw up charts, submit emails, have doctors or teachers testify on your behalf, do it. Make it a concise case with a lot of visual impact and don't stray from the main point(s).