Background It is critical to identify children living with HIV and initiate lifesaving treatment early. The Pediatric Accelerated Case Finding Effort focused on line-listing and HIV testing of biological children among HIV-positive clients. We describe HIV testing and seropositivity rates following the initiative and gaps along the pediatric index testing cascade to inform HIV case finding optimization. Methods This mixed-methods study involved collecting monthly data on index testing outcomes, including line-listing (identifying biologic children < 15 years), HIV testing and ART initiation from March 2020 to July 2021 in 35 facilities in Kinshasa. Data were summarized and presented for the first month (as a baseline proxy) and the entire study period. Qualitative data were collected from 14 healthcare workers participating in in-depth interviews and 33 community health workers in four focus group discussions. Audio recordings were transcribed and translated from Lingala or French into English and coded using MAXQDA software. Data were thematically analyzed according pediatric case finding barriers and strategies. Results At baseline, in March 2020, among 3,337 eligible female index clients, 1,634 (49.0%) underwent line-listing to identify children with unknown HIV status. By July 2021, all eligible female index clients (n = 11,734) had a line-list. Of the contacts, 9,871/11,848 (83.3%) were HIV-tested. Of contacts tested, 662 (6.7%) were diagnosed as HIV-positive, with 535 (80.8%) age 5–14 years; 99.5% initiated treatment. Providers attributed gaps in HIV testing primarily to testing refusals for children due to non-disclosure among parents and logistical or financial obstacles to transportation for outreach. COVID-19 movement restrictions and exposure fears also limited provider interactions for testing. Provider-implemented strategies included transport reimbursement, extensive counseling and alternative approaches to child testing for parents in sero-discordant relationships. Conclusion Following intensified efforts around pediatric case finding, we found a high HIV positivity yield of 6.7% among previously undiagnosed children, with 81% of infected children ≥ 5 years. While line-listing improved over time, outreach for HIV testing remained the largest gap, missing an opportunity to reach 17% of undiagnosed children. Ensuring adequate resources for HIV testing and outreach and supporting disclosure among couples, while emphasizing elicitation of ART clients’ biological children can help to optimize pediatric case finding.