You have a JSON string, not an object. Tell jQuery that you expect a JSON response and it will parse it for you. Either use $.getJSON instead of $.get, or pass the dataType argument to $.get
:
$.get(
'index.php?r=admin/post/ajax',
{"parentCatId":parentCatId},
function(data){
$.each(data, function(key, value){
console.log(key + ":" + value)
})
},
'json'
);