How can I help a mother who is very disturbed about being in quarantine and away from her 3 year old child?

It is usually challenging when there is a young child involved. It would be helpful to allow the mother to express how she is feeling (helpless, worried, sad, etc.) in this situation and validate that it would be understandable to feel that way.    Thereafter, you could help her to identify the most meaningful, fulfilling and feasible way she can keep contact with her child while she is in quarantine (speak, sing or tell a story to the child, etc. over phone at times when the child might need her the most – meal times, bed time, etc.)   Help the mother to see that the child will manage her difficult feelings based on how the mother is managing her own. Hence, the mother could be encouraged to communicate calmly and positively with the child.   Also encourage the mother to identify ways in which she can keep herself motivated and hopeful during the quarantine period.