add_filter(
    'gform_field_validation',
    function( $result, $value, $form, $field ) {

        $form_config[71] = [  // Replace with your parent form ID
            'nested_field_id' => 1,        // Replace with fieldID of nested form
            'child_field_id'  => 1,      // Replace with your child form field ID
            'required_value'  => 'test',// Replace with the string value to validate against
        ]; //copy this with new id's for new forms

        $form_id = rgar( $form, 'id' );
        if ( in_array( $form_id, array_keys( $form_config ), true) ){
            if ( $field['id'] === $form_config[$form_id]['nested_field_id'] ) {
                $child_entry_ids = explode( ',', $value);
                $child_entries = [];
                foreach ($child_entry_ids as $child_entry_id) {
                    $child_entry = GFAPI::get_entry( intval( $child_entry_id ) );
					if ( ! is_wp_error( $child_entry ) ) {
						$child_entries[] = $child_entry;
					}
                }
                $child_field_id = $form_config[$form_id]['child_field_id'];
                $required_value_found = false;
                $required_value = $form_config[$form_id]['required_value'];
                foreach ( $child_entries as $child_entry ) {
                    if ( $child_entry[$child_field_id] === $required_value ) {
                        $required_value_found = true;
                        break;
                    }
                }
                if ( ! $required_value_found ){
                    $result['is_valid'] = false;
                    $result['message'] = "Validation failed. At least one entry should contain $required_value.";
                }
            }
        }

        return $result;
    },
    10,
    4);