Weight lifted by children or Mark (5)
I believe the answer is:
notch
'mark' is the definition.
(a notch is a mark or scratch on something)
'weight lifted by children' is the wordplay.
'weight' becomes 'ton' (unit of weight).
'lifted' is a reversal indicator (going up in down clue).
'by' means one lot of letters go next to another.
'children' becomes 'ch' (genealogical abbreviation).
'ton' backwards is 'not'.
'not'+'ch'='NOTCH'
'or' is the link.
(Other definitions for notch that I've seen before include "Incision; indentation" , "Incision on an edge" , "Point or degree in a scale" , "Nick - incision" , "Secure" .)