Okay, as for not being able to see the field in Sandbox, perhaps your APEX explorer is set to log into your production instance? If you go to Tools -> Options, is your Endpoint pointing to https://www.salesforce.com/services/Soap/u/14.0 or is it pointing to https://test.salesforce.com/services/Soap/u/14.0? If it's pointing to test, then you are looking at your sandbox version in Apex Explorer

As for your problem I hope this is what you were looking for

trigger checkHighestRecord on customAccountObject__c( after insert ) { List< customAccountObject__c > caoListToUpdate = new List< customAccountObject__c >(); for( customAccountObject__c cao : Trigger.new ) { // Get all the related custom objects associated to the account // I'm ordering the results by ID desc because the newest ID's should contain the largest autonumber // This way you can set the first result to be checked while you uncheck the rest of the results List< customAccountObject__c > caoList = [select id, Account__c, autonumber_field__c, check_if_highest_number__c, other_fields__c from customAccountObject__c where Account__c = :cao.Account__c order by id desc]; for( Integer i = 0; i < caoList.size(); i++ ) { // Set the first one to be checked. all the rest are false if ( i == 0 ) caoList[i].check_if_highest_number__c = true; else caoList[i].check_if_highest_number__c = false; } // Put the updated results into a list to be updated later caoListToUpdate.addAll( caoList ); } // Update the items update caoListToUpdate; }