If I do the math correctly, I'm not sure a CTR is needed here.
Y - Deposited $5,000 benefit $5,000
X - Benefited from Y's $5,000 deposit and Z's $4,000 deposit = $9,000.
Z - Deposited $8,000 Benefited from $4,000.
If X is the custodian for Z, X does not have ownership of these funds and therefore would not benefit from them. Consequently, no single person received more than $10,000 and no single individual deposited more than $10,000.
(The normal caveat to this answer is to of course, make sure that you are looking at these accounts for potential structuring.)
If, on the other hand, X and Z have a joint account and therfore X benefited from $12,000, aggregation rules would dictate that all joint owners are included so you would have 3 section A's and Section B would be left blank with the "Conducted on own behalf" box checked.
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