Line of children seeing potential danger in water (9)
I believe the answer is:
crocodile
I believe this is a double definition.
'line of children' is the first definition.
(crocodile can mean a line of school children)
'potential danger in water' is the second definition.
'seeing' acts as a link.
(Other definitions for crocodile that I've seen before include "Recoil, doc, from the reptile" , "Old Cicero, oddly, was a voracious reptile" , "Line of children walking in twos" , "Vicious beast" , "Large reptile with long jaws" .)