If you are querying
customobj__c and its child relationship
childrecs__r, the correct syntax is:
SELECT Id, <any other parent record fields>,
(SELECT Id, <any other child record fields>, cusfield__c FROM childrecs__r)
FROM customobj__c WHERE Id = '<parent id goes here>'
The convention is usually that the child relationship has a plural name. If you're not sure what it was set to, double-check by going to the child object in Setup and viewing the lookup field to
customobj__c to see what was set as the child relationship API name (not the field API name) for that lookup.
Or if you need only the children:
SELECT Id, <any other child record fields>, cusfield__c
FROM <child object name>__c
WHERE <parent relationship name>__c = '<parent id goes here>'