They went after Walsh for one reason: The parents are by FAR the ones most likely to murder children by overwhelming statistics. That's just good police work. Saying that they looked at Walsh doesn't mean that they let the real killer get away.
In fact, Walsh is the first person to say (I've heard him say this at least 10 times) to parents in this situation that they are under suspicion, to willingly take lie detector tests immediately without complaining so that they can be ruled out as supects.
I agree, look at the parents first. But the police work in Walsh's case wasn't good.