Background and context. Non-formal learning for K-12 computing education enables young people to learn about computing outside the formal curriculum. Many studies have reported on non-formal initiatives but it is not always clear what children and young people have gained from their participation. Objectives. This study set out to investigate non-formal learning initiatives by means of a systematic literature review. The two research questions addressed by the study are: (1) What has been the focus of recent computing education research about K-12 initiatives for young people and (2) What is the impact of non-formal K-12 computing initiatives? Method. A systematic literature review of computing education research was conducted, focused on non-formal initiatives for young people. Research was included from any country, but must be published in English between January 2015 and April 2021. Searches using key terms were performed across three databases. 88 studies were synthesised from over 400 initial results. Findings. The vast majority of studies reported on immersive multi-day settings such as summer camps run by universities (n= 67), with fewer (n=21) reporting on regular ongoing after-school or weekend clubs. The most popular affective outcomes measured by studies were self-efficacy (n=25) and interest (n=22). Measurement of cognitive outcomes, such as knowledge (n= 13) and skills (n=17), was less prevalent. 22 different topics were identified from the studies, with most studies being programming-heavy. The majority of papers measured the short-term impact of these interventions, and generally there was an inconsistent or incomplete reporting of learner characteristics across the studies. Implications. The lack of papers investigating regular after-school initiatives suggests that the majority of non-formal learners are not being studied or that summer school findings are being wrongly extrapolated to this setting. More rigorous research is needed for regular after-school and short-term non-formal contexts to ensure that this set of learners' experiences is understood and potentially improved.