Background The purpose of this study was to evaluate the longitudinal validity of the Dutch version of the Caregiver Priorities and Child Health Index of Life with Disabilities (CPCHILD), a health-related quality of life instrument for nonambulatory children with severe motor disabilities and accompanying disorders. Methods The effect of two interventions, Botox injections in the hip region and percutaneous endoscopic gastrostomy, was followed over time. Caregivers (n = 38) of nonambulatory children (26 boys, 12 girls; mean age: 9 years, 5 months [4 years, 9 months]) with severe disabilities completed the questionnaire prior to the intervention, at 3 months and 6 months follow up. Seven a priori hypotheses were formulated. Longitudinal validity was analysed by a paired t test of the pre-post scores and correlation analysis between the change-scores and two external criteria: a caregivers' perceived change in health-related quality of life of the child questionnaire and a general health-related quality of life instrument. Results The results reported here follow completely the pattern we hypothesized for four analyses and partially in the remaining three. In the Botox group, the mean change-score at 3 months was 6.9 points (p <0.05), which exceeds the minimal clinically important difference of 5.8 points. At 6 months, the effect was diminished to 4.5 points, in line with the temporary effect of Botox. There were moderate positive correlations between the change-scores and an external criterion (Spearman's rho: 0.46-0.58). Conclusions This study indicates that the CPCHILD Dutch version has sufficient longitudinal validity statistically and clinically in nonambulatory children with severe disabilities.