This feature is available in all editions.
This can occur if all three child projects use the same sprint schedule.
You can assign a unique sprint schedule for any project within the project tree. The project schedule setting can be changed under certain conditions and will un-assign any open items scheduled in a sprint/iteration. You can make this change by editing the Sprint Schedule assigned to the project in the Project details.
Editing and removing a project's sprint schedule requires the following:
You must have sufficient Project Administration rights to access the project
The project cannot have any closed backlog items, defects or portfolio items scheduled in a sprint/iteration. To verify this, check the following page.
Product Planning > Backlog
The project cannot have any closed retrospectives in the sprints being changed.