Child’s character we think encapsulates “untrustworthy” (11)
I believe the answer is:
treacherous
'untrustworthy' is the definition.
(I've seen this before)
'child's character we' is the wordplay.
'child's' becomes 'treacher' (I am not sure about this - if you are sure you should believe this answer much more).
'character' becomes 'o'.
'we' becomes 'us'.
'treacher'+'o'+'us'='TREACHEROUS'
'think encapsulates' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for treacherous that I've seen before include "covered in ice?" , "Perfidious" , "Disloyal; dangerous" , "Not to be trusted" , "Ready to betray" .)