Edge lifted on blue box to reveal something fun for children (6,5)
I believe the answer is:
pillow fight
'something fun for children' is the definition.
(children might have a pillow fight)
'edge lifted on blue box' is the wordplay.
'edge' becomes 'lip' (lip is a kind of edge).
'lifted' shows that the letters should be reversed in order (going up in down clue).
'on blue' becomes 'low' (I've seen this in other clues. I am not sure about the 'on' bit.).
'box' becomes 'fight' (boxing is a kind of fighting).
'lip' backwards is 'pil'.
'pil'+'low'+'fight'='PILLOW-FIGHT'
'to reveal' acts as a link.
(Other definitions for pillow fight that I've seen before include "Mock battle in bedroom" , "Dormitory battle" , "Bedroom conflict" , "Bedroom battle (where feathers fly?)" .)