Background: Acute kidney injury (AKI) is increasingly recognised in children with acute COVID-19 and multisystem inflammatory syndrome in children (MIS-C), yet the long-term renal consequences in younger paediatric populations remain unclear. Most studies focus on acute illness or mixed-age cohorts, with limited data specific to children aged 0–12 years. Objectives: This study aimed to systematically identify, evaluate, and synthesise evidence on post-acute (≥30 days) and long-term (≥90 days) kidney outcomes following SARS-CoV-2 infection or MIS-C in children aged 0–12 years, including chronic kidney disease (CKD), eGFR decline, proteinuria, haematuria, hypertension, and need for kidney replacement therapy. Methods: We searched MEDLINE, Embase, CINAHL, and PubMed (December 2019–30 November 2025), following PRISMA 2020 guidelines and a registered PROSPERO protocol (CRD420251241949). Observational studies reporting kidney outcomes ≥30 days post-infection in children aged 0–12 years were included. Risk of bias was assessed using the Newcastle–Ottawa Scale or ROBINS-I. Owing to heterogeneity and absence of ≥3 comparable datasets, a narrative synthesis was performed. Results: Seven studies met inclusion criteria (five MIS-C cohorts, two acute COVID-19 cohorts). Only a subset provided extractable data specific to children aged 0–12 years. Follow-up ranged from 30 days to 12 months; four studies reported outcomes ≥ 180 days. Across all studies, no incident CKD, sustained eGFR decline, or kidney replacement therapy were reported among children completing long-term follow-up; however, most long-term outcome data were derived from MIS-C cohorts with median ages around 8–11 years that included some adolescents, rather than exclusively children aged 0–12 years. One MIS-C study reported long-term hypertension in 14% of children. A cross-sectional Italian cohort of mild COVID-19 demonstrated hyperfiltration, proteinuria, and microhaematuria at ~3 months, though chronicity could not be assessed due to absence of baseline values. A large US EHR-based cohort identified increased CKD risk after COVID-19 in the broader < 21-year population; however, 0–12-year-specific event counts were not reported, preventing quantitative synthesis for young children. Conclusions: Evidence on long-term kidney outcomes after SARS-CoV-2 infection in children aged 0–12 years remains limited, and only a small subset of studies provided extractable, age-specific data. On the other hand, MIS-C cohorts generally show favourable renal recovery, small sample sizes, lack of control groups, and short follow-up restrict confidence in these findings. Large paediatric EHR studies suggest potential long-term renal risk in broader paediatric populations, highlighting the need for age-stratified, prospective cohorts with serial eGFR, urine studies, and blood pressure assessments. Until definitive evidence emerges, structured renal follow-up may be warranted for children with AKI or MIS-C during COVID-19.