In a master-child setup, ack events on child ar not available in master.
From master logs:

6961:20140913:202816.555 NODE 1: received history from node 2 for node 2 datalen 4806
6964:20140913:202817.129 NODE 1: received history_uint from node 2 for node 2 datalen 4112
6963:20140913:202819.733 NODE 1: received events from node 2 for node 2 datalen 112472
6964:20140913:202820.861 NODE 1: received acknowledges from node 2 for node 2 datalen 1452
6964:20140913:202820.862 [Z3005] query failed: [1452] Cannot add or update a child row: a foreign key constraint fails (`zabbix`.`acknowledges`, CONSTRAINT `c_acknowledges_2` FOREIGN KEY (`eventid`) REFERENCES `events` (`eventid`) ON DELETE CASCADE) [insert into acknowledges (acknowledgeid,userid,eventid,clock,message) values (200000000000001,200200000000001,200000000000050,1409656949,'ok'),(200000000000002,200200000000001,200000000001685,1409768970,'k'),(200000000000003,200200000000001,200000000002229,1409776314,'not critical yet.'),(200000000000004,200200000000001,200000000002389,1409824126,'talked with devs to clean their homes.'),(200000000000005,200200000000001,200000000002368,1409824137,'cleaning up.'),(200000000000006,200200000000001,200000000002369,1409824146,'cleaning up.'),(200000000000007,200200000000001,200000000002390,1409824233,'due to disk cleanup.'),(200000000000008,200200000000001,200000000002394,1409830010,'will leave as is, we can increase the machine size once we know what the monitored pool is.'),(200000000000009,200200000000001,200000000002394,1409830011,'will leave as is, we can increase the machine size once we know what the monitored pool is.'),(200000000000010,200200000000001,200000000002555,1410337150,'deploy update'),(200000000000011,200200000000001,200000000002538,1410337156,'deploy update'),(200000000000012,200200000000001,200000000002598,1410512426,'update.'),(200200000000001,200200000000001,200000000002743,1410632917,'testing');
]

Thank you.