Using the Combo Blocks plugin, you can easily query posts by parent for Post Grid or Post Grid – Filterable Gutenberg blocks.
To begin, insert the “Post Grid” or “Post Grid – Filterable” block.

From the List View make sure you’ve selected Post Query nested block. This selection is important to access all the available block settings for “Post Query”.

Now under Query Post => Add Query Params select the Post Parent option.

The post parent dropdown tab is accessible by selecting the post parent. Now click and enter the Parent Page ID. The entered Parent Page ID will change your Post Grid.

That’s how you can query posts by parent for “Post Grid” or “Post Grid – Filterable” Gutenberg block. If you still need help, please create a support ticket on our forum.