This is now supported in the Application Builder using the 'Custom Single and Multiple Answer Questions' Feature Preview.
So what you're looking to do is create a multiselect that lists cases to be selected? Unfortunately this is not supported in CommCareHQ's Application Builder. It can be done through a custom method described below, but is not recommended.
The process described below is not actively supported by CommCare
This feature requires a software plan
This is useful if, for example, you have a form where you want to check off which of your cases are attending.
Modifying the XForm
<instance id="casedb" src="jr://instance/casedb"/>
<label ref="jr:itext('yourquestion-label')" />
<label ref="jr:itext('yourquestion-label')" />
Variations
If you would like to restrict the cases shown to be only child cases of the case which you are updating you can insert the following syntax as above:
[index/parent = instance('commcaresession')/session/data/case_id]
For Dimagi people reference:
You can look at this app for reference: https://www.commcarehq.org/a/tdhtesting/apps/view/39d3a2c79d16e59d035a4093fa0cf3cd/modules-1/forms-2/source/
In Child Classification form, the 'Authentication' group of questions at top. To see how access case properties of the selected case, look at hidden variable 'selected_user_pin'