Edit in JSFiddle

var myObj = {
  child1: "data1",
  child2: "data2",
  child3: "data3",
  child4: "data4"
};

var a = (function addAfterChild(data, trigChild, newAttribute, newValue) {
  var newObj = {};
  Object.keys(data).some(function(k) {
    //iterate through the keys and keep pushing them to the new object
    newObj[k] = data[k];
    if (k === trigChild) {
      //if the selected attribute is found, push the new value
      newObj[newAttribute] = newValue;
    }
  });
  myObj= newObj;
})(myObj, "child2", "NEW_KEY", "NEW_VALUE");

document.getElementById("result").innerHTML = JSON.stringify(myObj);

<p>
  Result after adding new attribute after "child2":
</p>
<p id="result">

</p>