Hi!

I hope I'm understanding your question right, I think you're talking about something that could be achieved with the Listen-to-Widget functionality. This requires a few steps but should be possible to set up. 

1. Put a Data view on your page (you can find this inside the Toolbox on the right side of your modeler) or if you already have a Data View for your Child table, you can skip to Step 3. We'll connect the Data Source later on.

2. Put the Child Table inside the Data view, if you haven't already, add the attributes you require dynamically via the Data View parameter so you have the right information on the page. 

3. Make sure the Parent Table is changed to a Datagrid 2 / List etc. so you can see a list of parent items on the page and set a selection method to make them clickable (See image below for Datagrid 2) - (I prefer using the Datagrid 2, when using a DG2 make sure to set the following properties (it won't show up in step 5 otherwise, as no selection or multiselection won't return a single object, and that's what you need when using the Listen to Widget):

4. Now for the magic: Go back to the first Data view that's holding the ChildTable. Open the properties panel and make sure to enable the Listen to widget setting:

5. In the List widget dropdown you should see your datagrid (if it's set up properly).

6. Select it, and run the app. Now when you click on your parent Datagrid item or List item, your ChildTable should change the data to that of the parent object you selected in the DG2 or List item.

You can find more about the Listen to Widget in the Mendix docs.

In the Academy there's a path where they explain the use of the Listen to Widget as well: https://academy.mendix.com/link/modules/605/lectures/4710/3.5.1-Add-the-Translate-Button

They kinda skim over it quickly but there is some explanation on it. It might be good to check that one out :)!

Hope it helps, if I got your question wrong feel free to reach out with a bit more info!

Have a great day,

T.