Every time in the past when a human is exposed to Terrigen mist it is fatal (Agent Tripp) or near-fatal (Agent Coulson, saved only by amputating his hand). Yet here three human children are exposed and survive.
It's not the Terrigen mist that is harmful, it was the metal from the "Diviner" that was harmful. Jiaying (Daisy's mother) was afraid that since there were only 4 Diviners remaining on earth, that if they were ever lost or destroyed, the Inhuman race would go extinct. So she had asked the (Inhuman) scientists to duplicate the Terrigen crystals. They were successful, only they were unable to make them without the Diviner metal mixed in with it, making the new crystals deadly to all who were not Inhuman. The Kree however have the original crystals, with no Diviner metal mixed within. So that's why the remaining 3 children who were exposed survived.
