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If you're dealing with an older teen who is asking you for reunification with your ex, then yes, you may speak to your child and explain what had led to the separation. You may chose to omit some examples and still give an idea of what had transpired and what you've had to deal with. It may have also been a healthier decision to separate at that time in order not to expose your child to unhealthy family dynamics.
Try to find out what your child's understanding is about what had led to the separation, ask them under what circumstances would they leave a partner even if they are married with child. Explain that adults can find differences b/w each other that make their life together more unhealthy than if they separated. Separation in that case shows maturity and responsibility that your teen may have to learn is a part of a marriage as well as of the decision to split up. Address the fact that you understand that it may have been difficult to live without both parents being together and that nevertheless, both parents (if your ex is involved in the teens life) had been offering a healthier situation to their child than should they have remained together.