Preschoolers are already copying the adults of the same sex as them. If the child relates to the person who uses abuse they may be aggressive to others including the abused parent. If they relate to the person who is abused they may be fearful and anxious.
They may also be unable to trust adults and not build attachment with parents. In some cases the opposite will happen and they may very be insecure and seek attention and comfort even from strangers.
Children of this age may also believe they are responsible for the abuse between the adults and that they are bad or stupid.
In cases where the abuse started when the child was a little older the child may begin bed-wetting or thumb sucking. The child may be afraid to sleep alone or lose some of their language development, resorting baby-talk.