In analysis of Child Health data, generally z-scores or percentile groupings are used as children do not growth is not linear.
The CDC (Center for Disease Control and Prevention) have released tables of data for calculating these z-scores and percentiles, and here are some scripts for R to calculate these in your sample.
Update – Nov 2013
The site I was hosting the code on closed, scripts are now available on my github.
Note: these are reasonably straight forward, please check the code. Age can be a ‘tricky one’, given rounding and cut points etc so you’re encouraged to double check your results.