Introduction: Discontinuation of tuberculosis treatment (DTT) among children in sub-Saharan Africa is a major obstacle to effective tuberculosis (TB) control and has the potential to worsen the emergence of multi-drug resistant TB and death. DTT in children is understudied in Uganda. We examined the level and factors associated with DTT among children at four large health facilities in Kampala Capital City Authority and documented the reasons for DTT from treatment supporters and healthcare provider perspectives. Methods: We conducted a retrospective cohort analysis of records for children <15 years diagnosed and treated for TB between January 2018 and December 2019. We held focus group discussions with treatment supporters and key informant interviews with healthcare providers. We defined DTT as the stoppage of TB treatment for 30 or more consecutive days. We used a stepwise generalized linear model to assess factors independently associated with DTT and content analysis for the qualitative data reported using sub-themes. Results: Of 312 participants enrolled, 35 (11.2%) had discontinued TB treatment. The reasons for DTT included lack of privacy at healthcare facilities for children with TB and their treatment supporters, the disappearance of TB symptoms following treatment initiation, poor implementation of the community-based directly observed therapy short-course (CB-DOTS) strategy, insufficient funding to the TB program, and frequent stock-outs of TB drugs. DTT was more likely during the continuation phase of TB treatment compared to the intensive phase (Adjusted odds ratio (aOR), 5.22; 95% Confidence Interval (CI), 1.76-17.52) and when the treatment supporter was employed compared to when the treatment supporter was unemployed (aOR, 3.60; 95% CI, 1.34-11.38). Conclusion: Many children with TB discontinue TB treatment and this might exacerbate TB morbidity and mortality. Treatment supporters of children with TB should be provided with detailed and consistent health education messages about TB treatment duration, the benefits of treatment completion, and risks of treatment discontinuation. The district and national TB control programs should address gaps in funding to TB care and the supply of TB drugs.