The inability of children to comprehend and process death due to factors like chronological age, attachment of the child to the deceased, nature of death, and family support, might lead to delayed or prolonged grief and consequently complicated grief (CG). Complicated grief in a child’s life can affect his/her social and cognitive functioning manifesting in antisocial behaviours. This study aimed to determine the prevalence of complicated grief among children who had lost loved ones in selected public primary schools in Nairobi County. This study was guided by Piaget’s Cognitive Development Theory. Multistage sampling, purposive sampling, inclusion, and exclusion criteria were used to select 259 pupils aged 10-13 years who had lost a loved one in the last year. Purposive sampling was also used to select 22 class teachers of the bereaved pupils who participated in the study. The study employed a convergent mixed-method design. Data was collected through self-administered questionnaires (SDQ, ICG, and STAB) and interviews. Quantitative data was analyzed through descriptive and inferential statistics using SPSS Version 25.0. The findings indicated that the prevalence of complicated grief was 26.1%. The study concluded that complicated grief was prevalent among children who had lost a loved one. Therefore, the study recommends that the Ministry of Education introduce a school-based counseling program incorporating grief intervention techniques in public primary schools. During grief, a child’s world changes unexpectedly from conversant, predictable, and safe, to chaotic and dreadful. Parents and caregivers are also implored to brace up and talk about death to children, and involve them during funeral preparations.