We sometimes forget or are unaware of what children may have going on at home. Our priority is (as it should be) providing the best possible educational opportunities. In order to do this, we need to be aware if primary needs of the child are being met. In young children, the term "the reptilian brain" has been used to explain that focus on school and other activities will be lessened if energy is used worrying because the child does not feel safe and provided for. This priority of energy focus happens at all ages if basic survival feels threatened.
School programs who offer breakfast and reduced or free meals to children are meeting nutritional needs. Our own school system, I am proud to say, participates in programs providing weekend backpacks of food and summer meals for some children.
We also provide transportation from domestic violence shelters to children's home schools, so they have that familiar environment instead of having to switch schools with all the other changes in their lives.
Also keep in mind that everything is relative. Even a child who has not experienced the horrors of family violence may be very sensitive and affected by things he perceives in a family. Very young children may misunderstnad tension or a conversation and internalize worry they don't even understand and cannot explain. Some children just have more anxiety than others and can be very distracted by problems with jobs, money, relationships, or other perceived concerns with the family. You also may not be aware of close relatives and friends that may be sick or have even passed. Children can experience extreme grief over the loss of a pet also. Having time to get to know your students not only heightens your awareness of what is going on in their lives, but makes them feel cared for.
Be aware and sensitive while helping the student feel safe, provided for, and cared about. This will help them feel more comfortable and improve their attention, but it may still be difficult for that child to focus all the energy necessary to do as well as they can. Prioritizing and reducing work for a time may allow them to keep up with what they must know and feel success. Keep in mind the principles of Understanding by Design (UbD), also known as backward design. Curriculum is designed based on desired outcomes. Decide what children must know, understand, and be able to do for the long term before designing lessons to teach. Keep this principle in mind as you prioritize requirements for children in differing levels of crisis.