For children under 5, the general recommendation is to use the haz, waz, and whz scores from the WHO 2006 guidelines, and calculate stunting, underweight, wasting, and overweight from those scores, rather than using the BMI. The BMI is recommended mainly for adults. However, you can calculate the BMI using the following lines in Stata, after you have opened the PR file. For women, weight is ha2, height is ha3, and BMI is ha40. For children, weight is hc2 and height is hc3, and I will use hc40 for BMI. If you are using the KR file, weight is hw2 and height is hw3. You will get the BMI, hc40, with a factor of 1000, with these lines:

gen loghc2=log(hc2)
gen loghc3=log(hc3)

* factor is 10 million, i.e. 1 followed by 7 zeroes
scalar factor=10000000

gen loghc40=.
replace loghc40=log(hc2)-2*log(hc3)+log(factor) if hc2<9000 & hc3<9000
gen hc40=exp(loghc40)
replace hc40=9998 if hc40>6000 & hc40<.
drop log* 

Please let me know if this does not seem to work.