Physically, mental abuse in young children impacts their brain development. The nerves themselves fail to develop into their full potential and subsequently so does the child. They will not have the neurological infrastructure to develop healthy relationships, learn to their maximum capacity, handle stress in healthy ways, and many et ceteras.
As an observer, mental abuse in a child is evidenced by a high-strung, defensive, moody, aggressive or withdrawn youngster with little confidence in themselves and little interest in socializing or group activities. They may be self-deprecating or act like a bully to others to cover their own feelings of inadequacy.
Without something in the child's life to buffer the abuse, the effects will continue. However, if there is a loving, nurturing relationship to offset the abusive one, the impact will be less severe.