It has been examined, what happens to small children (age zero to three), who have been traumatized following cancer. Small children’s symptoms of PTSD have been found to be different from that of adults, why alternative diagnostic criteria has been suggested. The individual experience of the situation as traumatic is a result, of the complex interaction between subjective and objective factors. Therefore, a broad assessment, involving a discussion of multiple factors affecting the small children’s experience of, and reaction to trauma, has been implemented. It seems especially important, to take developmental and parental support into consideration. The effects of trauma on small children’s development have been examined from different theoretical perspectives. It has been found, that trauma has cognitive, emotional, and social developmental consequences.