The wealth index file for the Burkina Faso 1998 survey does not match to v001 and v002. Instead the variables match to the variables sdepart (departement) sconcess (concession) and snumber (household number within concession). You could extract the equivalents of these variables as follows:
gen str3 wdepart=substr(whhid,5,3)
gen str3 wconcess=substr(whhid,8,3)
gen str2 wnumber=substr(whhid,11,2)
destring wdepart, gen(sdepart)
destring wconcess, gen(sconcess)
destring wnumber, gen(snumber)
drop wdepart wconcess wnumber
However, a much simpler method is just to create whhid in the children's dataset and to merge the wealth index file to the children's dataset using:
gen str12 whhid=substr(caseid,1,12)
merge m:1 whhid using BFWI31FL.dta