trigger AutoCreateCapturePlan on Project__c (after insert) 
{
    List<SYS_Capture_Plan__c> SYSCapturePlan = new List<SYS_Capture_plan__c>();
     
    for (Project__c newPosition: Trigger.New) {
         {
            SYSCapturePlan.add(new SYS_Capture_Plan__c(
                        Name = newPosition.Name + ' - Capture Plan',
                        Project__c = newPosition.id ));
        }
    }
    insert SYSCapturePlan;

    for(SYS_Capture_Plan__c cpc : SYSCapturePlan)
    {
	Project__c project = new Project__c(id = cpc.Project__c);
	project.Capture_Plan_id__c = cpc.Id;
	lstProjectsToUpdate.add(project);
    }

    update lstProjectsToUpdate;
}

 Hi!

This isn't exactly the right code, I just thought of showing you an example of how you can achieve what you want to do. Its not exactly right because the field where you'll be storing the Id's of child records should have some specific format. Like in case of multiple child records, how should the id be stored in that field?

About test coverage for this trigger,  all you need to do is create a test Project__c record with all the required fields and values(if there are any conditions) and insert it, rest will be taken care of.

Here all you would need is the Project Name and Id for the child record to get created :

so your test coverage can be :

@isTest
private class test {

    static testMethod void myUnitTest() {
        Project__c project = new Project__c(Name='test');// in case of required fields other than name, do //add them too.
        insert project; // once you perform this action, the trigger gets fired and creates the child        //record for the Project.
    }
}