Immature beasts ruin child's play (4,5)
I believe the answer is:
kids stuff
'immature beasts ruin' is the definition.
The definition suggests a singular noun which matches the answer.
'child's play' is the wordplay.
'child's' becomes 'kid's' ('kid' can be a synonym of 'child').
'play' becomes 'stuff' (I can't justify this - if you can you should believe this answer much more).
'kids'+'stuff'='KIDS' STUFF'
Can you help me to learn more?
(Other definitions for kids stuff that I've seen before include "A piece of cake" , "Something very easy" , "Something suitable for children (colloq.)" .)