You can't sue a judge for gross negligence.
But you can threaten both the father and his family with a civil suit of assault and battery. Perhaps a lawyer would take this on a contingency.
Couples with children should not be allowed to divorce, period.
There is NO way to "confirm" this with a 4 year old. A lot of these child psychologists (and the people who trained them) have an agenda, and use extremely leading questioning and props to get the answers they want out of the poor little kid. Have you ever seend the "anatomically correct" dolls they use? It's like one of those "put the round peg in the round hole" toys, only the gaping holes and huge "pegs" are so obvious and startling on a doll, that a young child quickly figures out what the shrink want him/her to do with them. By the time a young child has been through one of these brainwashing sessions, they're often really (and permanently) not able to distinguish what really did or didn't happen from the story they were led into telling -- just as young children often can't distinguish between something that really happened and a dream.
Unless this father has a confirmed history of this type of abuse (i.e. with real physical evidence and/or much older children), assume that it didn't happen.