Finally the forums work again (yes!) and I can reply.
It works! Thank you Bruno, if you send me your Paypal I'll buy you a beer!
At first I had some problems, but then I added my custom db prefix to
core/components/michelle/migxconfigs/childstutorial.config.inc.php
like this:
$prefix = $this->customconfigs['prefix'];
and changed this line:
$this->modx->addPackage($packageName, $modelpath);
to
$this->modx->addPackage($packageName, $modelpath, $prefix);
And it worked.
Now I'm trying to add Quip to the mix.
So far I've managed to show the name of a commenter in the childstutorial grid! (not very useful yet, but for me it's quite an accomplishment ;-)
First I added the package by adding the following code to my custom config file:
//quip
$packageName2 = 'quip';
$packagepath2 = $this->modx->getOption('core_path') . 'components/' . $packageName2 . '/';
$modelpath2 = $packagepath2 . 'model/';
$this->modx->addPackage($packageName2, $modelpath2, $prefix);
Then I altered the joins code for my childstutorial MIGX configuration to the following and this seems to work:
[{"alias":"rating","classname":"starRating","on":"star_id=modResource.id AND group_id=1"},
{"alias":"comments","classname":"quipComment","on":"resource=modResource.id"}]
Then in the Columns tab I added this:
Header: Quip comments
field: comments_name
show in grid: yes
And voila, the name of (I guess the last comment for a particular resource) shows in the grid of childstutorial! So cool

. Don't know if I did this the correct way so far.
I want to do the following now:
1) show the total amount of comments for a resource in the main childstutorial grid. I think the rows in the table for a particular resource ID should be counted, because in the modxs_quip_comments table there can be several entries for one resource. The resource ID is placed in the column 'resource'. Should I do this with a renderer or another way?
2) For each resource, when clicked on the 'edit' button, I want to show a tab called 'Comments'. Therefore I added a formtab 'Comments' on the childstutorial configuration (under Formtabs).
I thought I would have to create another migx configuration for the comments. This is where I get stuck, because how can I make sure that only the comments for a particular resource will be shown here? Basically I want the same configuration as is used for the Articles component, where you can update, approve, disapprove and delete comments. Do you think there is a way I can copy the configuration from the Articles component and use it for my own MIGXdb-tv?
Is the setup like it's done in Articles possible with MIGXdb anyway?