How can I stop myself from running down my ex-spouse in front of our child?

A child carries half of each parent, so running down your ex-spouse feels to the child as though half of themselves is being run down. However angry you are, a child should not carry adult wounds. When you feel the urge to badmouth, stop and remember that your child needs to love both parents without guilt. If your child asks difficult questions, answer without bringing down the other parent's dignity. Pour out your anger to a trusted adult or a counsellor, not to your child. Guarding your tongue in this is among the greatest gifts you can give your child. See educating the family.