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

    // $forms_and_fields[child form id]
	$forms_and_fields[66] = [
		//child field id => 'message to display',
		4 => 'message to display',
		2 => 'message to display',
        3 => 'message to display'
	];

	if ( in_array( $form['id'], array_keys( $forms_and_fields ) ) ) {
		if ( in_array( $field['id'], array_keys( $forms_and_fields[ $form['id'] ] ) ) ) {

			$nested_form    = new GP_Nested_Forms( $form );
			$parent_id      = $nested_form->get_parent_form_id();
			$parent_form    = GFAPI::get_form( $parent_id );
			$parent_form    = new GP_Nested_Forms( $parent_form );
			$session        = new GPNF_Session( $parent_id );
			$entry_id_array = $session->get_valid_entry_ids( $session->get_cookie()['nested_entries'] );
			$entries_array  = array();
			foreach ( $entry_id_array as $entry_id ) {
				if ( is_array( $entry_id ) ) {
					foreach ( $entry_id as $id ) {
						$entries_array[] = $id;
					}
				} else {
					$entries_array[] = $entry_id;
				}
			}
			$entries = $parent_form->get_entries( $entries_array );
            foreach ( $entries as $entry ) {
                if ( $field['type'] === 'number' && $value === $entry[$field['id']] ) {
                    $validation_result['is_valid'] = false;
                    $validation_result['message'] = $forms_and_fields[ $form['id'] ][ $field['id'] ];
                    break;
                } elseif ( $field['type'] === 'name'){
                    $field_id = $field['id'];
                    if ($value[$field_id.'3'] === $entry[$field_id.'3'] && $value[$field_id.'6'] === $entry[$field_id.'6']){
                        $validation_result['is_valid'] = false;
                        $validation_result['message'] = $forms_and_fields[ $form['id'] ][ $field['id'] ];
                        break;
                    }
                } elseif ( $field['type'] === 'date' ){
                    $value_time = strtotime( $value );
                    $entry_time = strtotime( $entry[$field['id']] );
                    if ( $value_time === $entry_time ) {
                        $validation_result['is_valid'] = false;
                        $validation_result['message'] = $forms_and_fields[ $form['id'] ][ $field['id'] ];
                        break;
                    }
                }
            }
		}
	}

	return $validation_result;
}, 10, 4 );