Previous research has shown that group processes are particularly pertinent to children's bullying, and who they socially exclude and include. This paper looks at how children's responses to social exclusion change according to their friends' group-based emotions. Children aged 8-11 years (N = 77) read stories about a friendship group to which they were said to belong and an instance of mild social exclusion. In the stories, the participants' friends' emotional reaction to the exclusion (pleased versus angry) was manipulated. Measures of assertive bystanding intentions and responses towards the friendship group and the social exclusion were taken. Children showed more assertive bystanding intentions when their friendship group was depicted as angry and they reported more anger when reacting to social exclusion. A mediation effect was found, with a perception of the friendship group's emotion as anger being related to increased assertive bystanding, through an increase in the participant's own anger towards their group's act of social exclusion. This study is among the first to show that from 8 years of age, the social appraisal of group emotions can account for children's reactions to social exclusion in a friendship group. Directions for future research in social appraisal of group-based emotion in social exclusion situations are discussed.