I think the percentile is the most frequent form to categorize the children's BMI. This WHO article could be interesting for you: http://eds.b.ebscohost.com/eds/pdfviewer/pdfviewer?sid=99769adb-9e04-4eac-909b-e690508cbbfb%40sessionmgr114&crlhashurl=login.aspx%253fdirect%253dtrue%2526profile%253dehost%2526scope%253dsite%2526authtype%253dcrawler%2526jrnl%253d00429686%2526AN%253d27143165&hid=103&vid=0