I think that the theme of this poem is, quite clearly, the impact that physical abuse has on women and children. The woman and the two kids in this poem have both clearly been abused and it has affected them.
The woman is, to me, distraught and afraid. It scares her when people ask about the blood on the baby. She is (presumably) afraid of what will happen if the police investigate.
The little girl is also affected. She seems not to really react to her mother in the way that a kid should. She is detached from her mother. Maybe it is because her mother cannot protect her from abuse. Maybe it is because she is so traumatized by parents in general. One way or another, the abuse has clearly affected her and made her unable to relate normally to her family.
The speaker establishes signs of abuse: the baby’s scabbed face, the little girl with her arm in a cast, the mother wearing sunglasses so that no one can see her eyes. Slightly more difficult as a sign of domestic violence is the mother’s body that “sags and tenses at the same time” (line 7). The speaker is writing a poem, not developing an analysis of domestic abuse; he/she seems to be dramatizing the situation, not preaching about it. This presentation creates the picture of the family, to which a reader will readily respond for the family as a whole. Terranova is here interested not in the theoretical aspects of family abuse, but in the actual reality of it.