(1) Background: The onset and duration of the child's illness is a difficult test for the entire family. The stress, in which the family lives from this moment, influences the relationships within it, as well as external relations with the environment. The child's cancer fundamentally changes the way the family functions, the quality of interactions within the family, and life plans. (2) Patients and Approach: A population survey was run between 2015 and 2020. A cross-sectional study was conducted involving 800 family caregivers of children with cancer during active treatment. The aim of the study was to assess changes in the family functioning in the opinion of parents of a child with cancer. (3) Results: Parents experience chronic anxiety (67%), nervousness (60%), and worry (64%). For 50% of parents, the illness of their child entirely changed their plans. As many as 75% of parents feel guilty for exposing their child to carcinogens. According to the parents, the child's illness is a psychological (89%), somatic (49%) and financial (77%) burden for them. Only 7% of children cause behavioural problems and 16% have trouble learning due to their sibling's illness. (4) Conclusions: Cancer is a great difficulty for all members of a family system. The disease disturbs the balance within the family and relations in the family, as well as more distant psychological, social, and material consequences.
Keywords: cancer; changes; children; family; parents.