Objective: To determine test–re-test reproducibility of the Timed Up & Go (TUG) test, 30-second repetition maximum (repmax) of functional exercises, 6-Minute Walk Test (6MWT) and High-level Mobility Assessment Tool (HiMAT) in children with Acquired Brain Injury (ABI). Secondarily, to assess the accuracy between hand-timed and video-timed scores for the TUG test and HiMAT. Methods: Thirty children at least 1 year post-ABI (mean age at assessment = 11 years 11 months, SD = 2 years 4 months; 14 males; Gross Motor Function Classification Scale I = 17, II = 13) were assessed twice. Intra-class correlation coefficients (ICC), standard error of measurement and minimum detectable change (MDC) were determined. The Bland-Altman method and 95% limits of agreement (LOA) were used to assess the agreement between hand and video-timed TUG test and HiMAT scores. Results: Test–re-test reproducibility was acceptable for the TUG test (ICC = 0.92; MDC = 1.2s); repmax of functional exercises (ICC = 0.84–0.98; MDC = 4–8 reps); 6MWT (ICC = 0.90; MDC = 69.38 m) and HiMAT (ICC = 0.98; MDC = 6). Comparison of hand and video scores for the TUG test and HiMAT demonstrated a mean difference of 0.23 (LOA = –0.3–0.7) and –0.07 (LOA = –1.99–1.85), respectively. Conclusions Test–re-test reproducibility of lower limb activity capacity measures in children with ABI are acceptable. The MDC scores provide a useful reference to interpret treatment effectiveness. Video timing was more accurate than hand-timing for the TUG test.