Children ultimately in pathetic fight for play area (7)
I believe the answer is:
Here is my best explanation:
'play area' is the definition.
'children ultimately in pathetic fight' is the wordplay.
'ultimately' suggests the final letters.
'in' means one lot of letters goes inside another.
'pathetic' becomes 'sad' (**).
'fight' becomes 'box' (boxing is a kind of fighting).
The final letter of 'children' is 'n'.
'n' put within 'sad' is 'sand'.
'for' is the link.
(Another definition for sandbox that I've seen is "It's full of grains!".)