Children, one vital force (3)
I believe the answer is:
chi
'vital force' is the definition.
(life force in Chinese philosophy)
'children one' is the wordplay.
'children' becomes 'ch' (genealogical abbreviation).
'one' becomes 'i' (Roman numeral).
'ch'+'i'='CHI'
(Other definitions for chi that I've seen before include "Classical character" , "The 22nd letter of Greek alphabet" , "Twenty-second Greek letter" , "Greek letter; force in Chinese philosophy" , "Greek letter; Chinese life force" .)