How to pass data from child custom element back to parent element from where it is called in Polymer 2

i have this custom element in polymer below

 <parent-element> 
     <child-element myData="{{data}}"> </child-element>   <!--modifies the data-->
     <!-- Want to pass updated data to child-element-2-->

     <child-element-2> </child-element-2> 
 </parent-element>

Let me know how to pass the updated data to 2nd child element

Data-binding is the way to go here.
You should be using a dash on the attribute while in the properties a camelCase. You can read about this in the Polymer documentation Property name to attribute name mapping.

<dom-module id="my-element">
  <template>
    <my-child-one data="{{data}}"></my-child-one>
    <my-child-two add-data="[[data]]"></my-child-two>
  </template>
  <script>
   Polymer({
     is: 'my-element',

       properties: {

         data: {
          type: String,
       },
   });
  </script>
</dom-module>

Also do not forget to set notify to true on the property inside the my-child-one Element.