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To chime in, I heartily agree with those suggesting a therapist for the poor daughter, as well as a united front with your ex reassuring her of both of your love and support.
In addition to that, though, is there a family friend--preferably one who hasn't chosen "sides"--that she can spend some time with? Someone she knows and loves? Someone she can talk to about the divorce, without having to tiptoe around her parents' feelings (or her perceptions thereof)? Someone who can just *listen* to her?
She needs all the love and support she can get, and that's before you add on that "Daddy" is going to become "Mommy."