DISCLAIMER: My following post is directed towards traditional heterosexual (straight) couples. I am in no way bashing homosexuals in any way with the theme of this post for I am not speaking to that demographic of people with this post. This is simply my opinion. **THIS IS LONG WINDED!**
A woman cannot teach her son how to be a man. A father does that. She can however protect him, teach him to be in touch with his feminine side (empathy towards others, caring, commitment, etc.) and serve as the example of the kind of woman he should want for himself and how to identify a woman worth respecting. A man cannot teach his daughter how to be a woman. A mother is for that. He can however protect her, teach her to be in touch with her masculine side (standing firm in her decisions, self esteem, identity) and serve as the example of the kind of man she should want for herself and how to identify a man worth respecting. Naturally they can teach their offspring of similar sex those exact traits, but from the perspective of how to ACT on it through actions. So I won't get into that. But this is only as effective as how the husband and the wife treat each other. They have to practice what they preach.
Many times I have commented about our natural roles in the household and how if one of those elements are missing long enough, it can throw things out of balance. Now in no way am I saying to rush into something. But be smart about it and understand why it's needed. There are some things a man cannot teach a woman, like there are some things a woman cannot teach a man. For the most part the attempt will be ineffective, and if effective will have opposite results. But who these role models are many times do not have to be the biological parent, though it is preferred that it SHOULD be.
It saddens me to see so many fathers NOT in their children's lives, only coming around when it's beneficial to THEM (Deadbeats). But at the same time it hurts for me to see women tell their children just how bad of a father he is when no attempt at a relationship was established between them (father and child) while fighting them in front of the children. Yet still sending them to their father after saying all of that. Both of those examples don't see how doing that is VERY counterproductive to the upbringing of that child.
If you insist on downplaying each other and their importance to their own children by telling them "your father ain't shit" or "your mother ain't shit" yet you insist on sending them to that person, it crosses wires and sends mixed messages to your children. I'm sure they're thinking "You keep saying this person ain't shit, yet it's perfectly fine to keep sending me to them. So am I not good enough for you to keep me away from them? Is there anyone better you can send me to to spend my time with?". They simply don't know how to communicate this with you yet, unless they're of age where they understand how to do so. They pay more attention than you realize. And if you insist on doing that to them, at least have another role model for them to look up to that falls in the paradigm of what a "good male/female role model" is and allow them to take it from there.
If you're separated allow your children to form their own opinion and relationship of and with their other parent. But remain open if they have questions and only interfere if something may be detrimental to some part, if not all, of their overall well-being or outlook on life. And when they ask those questions, tell them the truth, but not in a bashing way. Help them to understand what's going on and allow for them to grasp it, and if there's questions you can't answer admit you can't answer them, but send them to someone you would consider a "good role model" to get those answers. If not all then at least a sense of direction of where to find them.
And before you ask, no I don't have any children. I can speak on this because I am a product of this method. Trust it works.