BACKGROUND: Global child disability data are generally non-comparable, comprising different tools, methodologies and disability definitions. UNICEF and The Washington Group on Disability Statistics (WG) have developed a new tool on child functioning and disability to address this need. AIMS: The aim of this paper is to describe the development of the new module, and to present an independent field test of the draft module in two contrasting settings. METHODS: UNICEF and the WG developed a parent-reported survey module to identify children aged 2-17 years with functional difficulties in population-based surveys through: review of existing documentation, consultation with experts and cognitive testing. A field test of the draft module was undertaken in Cameroon and India within a population-based survey. Functional limitation in each of 14 domains was scored on a scale comprising "no difficulty", "some difficulty", "a lot of difficulty" and "cannot do". RESULTS: In all, 1713 children in Cameroon and 1101 children in India were assessed. Sixty-four percent of children in Cameroon and 35% of children in India were reported to have at least some difficulty in one or more domain. The proportion reported to have either "a lot of difficulty" or "cannot do" was 9% in Cameroon and 4% in India. There were no significant differences in reported functional difficulties by sex but children aged 2-4 were reported to have fewer functional difficulties of any kind compared with older children in both countries. CONCLUSION: Comparable estimates were generated between the two countries, providing an initial overview of the tool's outputs. The continued development of this standardised questionnaire for the collection of robust and reliable data on child disability is essential.