Prominent models of adult social anxiety disorder emphasize the role of hyperreactivity and emotion regulation (ER) difficulties. However, it is unclear whether these factors are relevant in childhood, a critical period for the development of this disorder. We used ecological momentary assessment with mobile phones to assess daily-life emotional reactivity and use and effectiveness of ER strategies in children aged 10–13 years. We compared three groups: Social anxiety disorder (n = 29), clinical controls with mixed anxiety disorders (n = 27) and healthy controls (n = 31). We also investigated long-term effects of ER on trait social anxiety 12 months later. Hierarchical linear modelling revealed higher emotional reactivity and more use of suppression in children with social anxiety disorder compared to clinical and healthy controls. Contrary to our expectations, children with social anxiety disorder reported more use of avoidance and reappraisal compared to clinical, but not healthy, controls. The groups did not differ in subjective effectiveness of ER strategies. Use of suppression, avoidance, and rumination each predicted an increase in social anxiety 12 months later. Taken together, our results extend previous findings from lab and questionnaire studies and illustrate the role of maladaptive ER for child social anxiety disorder.