Hi,
Please try with the below domain. Add it into the child product id field.

         domain="[('category_id', '=', parent.category_id)]"

Hope it will help you.
Up vote please.

Invalid composed field farm_materials.category_id in domain of ([('categ_id', '=', farm_materials.category_id)])

also

ValueError: Invalid composed field materials.category_id in domain of ([('categ_id', '=', materials.category_id)])