<apex:includeScript value="/support/console/34.0/integration.js"/>
    function openSTab(){
    var tid;
    var pg;
    var cid ='{!case.id}';
    if(sforce.console.isInConsole){
    sforce.console.getEnclosingPrimaryTabId(fetchTabId);
    }
    function fetchTabId(result){
    tid = result.id;
    pg = '/apex/ChildPage?id='+cid;
    sforce.console.getEnclosingTabId(openTab);
    }
    function openTab(result){
    sforce.console.openSubtab(tid,pg,true,'test',null,undefined,'test');
    }
    }

<apex:commandButton onclick="openSTab()"/>