Explosion, after which children all fall down? (7)
I believe this is a double definition.
'explosion' is the first definition.
(representation of the sound of a sneeze)
'after which children all fall down?' is the second definition.
(as in 'atishoo, atishoo, we all fall down')
(Other definitions for atishoo that I've seen before include "25 sound", "report of involuntary escape from trap?", "Sound made on expiration", "hay-fever sufferer may interject this", "Sudden eruption".)