
Originally Posted by
Masamune·1600
To clarify:
The children were under the impression that drinking the Black Water would cure them of their Geostigma. However, such was obviously not the case. Having taken them to the Forgotten Capital was sufficient to set a trap for Cloud; however, the actual Black Water was infused with Kadaj's will, and used to (presumably) activate the instinctual need for Reunion. The SHM most likely believed that, guided by that instinct, the children would be able to locate the JENOVA cells.
As to the matter of Geostigma, the condition is actually the result the internal conflict between a Gaian's innate Spirit Energy and JENOVA. Following the final chain of events in FFVII, when Sephiroth body was blown apart, it "infected" the Lifestream. When the Lifestream moved to Midgar to bolster Holy, the corrupted Lifestream tainted those that encountered it; hence, many people developed Geostigma. The condition did not exist prior to the end of FFVII.