When the fire ramp or khandak has been blessed by the Sajjads with their alams , and after they have walked on it, the public move in to walk the same , this is also known as Fire Matam..
This is done in the memory of the fourth Imam Zainul Abedin the surviving son of Imam Hussain in the karbala holocaust.
He along with the ladies of the house hold including little children, were manacled , chained, and made to walk bare feet to Sham Damascus over the burning sands.
This was Yazids form of torture, towards the family of the Holy Prophet, the massacre the genocide was uncalled for.
Yazid is and will always remain the Butcher of Karbala , walking his footsteps blood of this serpent seed was Saddam Hussain ... another modern time butcher of Shias and Kurds.
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About the kids walking on fire, well initially they are reluctant and scared , but they are never coaxed to walk, instead the volunteers lift them by the hands and let them cross over to safety..
But than there is always a first time , once the child gets over the fear he walks several times ...