Project visibility can be limited by person groups. Projects to be shown must be set for each person group, after which people assigned to the person group can see and target work to these projects.
Go Worktime > Administration > Person groups and choose 'Person group projects' from the Action menu.
Select the Person group which projects you want to manage.
Choose the wanted parent project from 'All projects' field. Giving visibility to parent project also gives visiibility to child projects. The selected projects move to the 'Selected projects' field.
Then press Save.