Actually, a few have been mentioned in previous threads. A father who was a cop killed his wife and the three year old son told police that daddy hurt mommy and she was in a rug.
There was also another case of a mother that was raped and murdered by the garbage man and her 3 year old daughter was left alive and was able to identify the man.
And quite a few cases where testimony from young children was just plain wrong and resulted in wrong people imprisoned. Basically, one can not rely on testimony from a young child for the most parts.