Previous works have shown that effective communication between parental caregivers and child patients has many benefits to the children, such as providing emotional support and coping skills for health management. Drawing on semi-structured interviews, we have identified the challenges parental caregivers face when communicating with their children about health-related information in daily illness management. Three salient challenges that the parental caregivers encountered include: (i) acknowledging different perceptions and approaches to being a cancer patient, (ii) choosing an appropriate communication method, and (iii) understanding their child's uncommunicated emotions. Based on these challenges, we recognize distinctive, yet implicit, needs that children develop during the illness trajectory, affecting the parent-child dyadic relationship. We discuss design opportunities for a collaborative system that enhances the parent-child dyadic communication by supporting the child's implicit and dynamically changing needs throughout the illness trajectory and beyond.