While the healthcare setting is commonly associated with some level of anxiety, this can extend to toxic harmful anxiety that has the potential to have a negative long-term effect on a child’s anxiety and coping mechanisms, if not addressed adequately by healthcare professionals. For healthcare professionals to intervene in a timely basis and address anxiety in children in hospital, it is important that they have valid and reliable tools that enable them to measure anxiety in the first place. The purpose of this study is to present the findings of the reliability tests of the Short-STAI to measure anxiety levels in children, comparing the results with the measurements derived from the other two reliable methods utilized in the same study which are the vital signs and the MYPAS (the Modified Yale Perioperative Anxiety Survey). The validity testing was carried out in an RCT that used the S-STAI, in which 88 surveys were completed by parents of children who had a tonsillectomy and adenoidectomy. All of the children were between 4 and 7 years of age. The relationships between variables were investigated and 25 showed a positive correlation between all items. Cronbach’s α for the entire scale was 0.79. This study has identified that the S-STAI is a reliable tool for parent report of a child’s anxiety in the peri-operative period.