This sounds like all three child projects were defined to use the same sprint (or iteration) schedule.

You can assign a unique sprint schedule for any project within the tree. The project schedule setting can be changed under certain conditions and will un-assign any open items scheduled in an sprint. The change is made by editing the Sprint Schedule assigned to the project on the Project Details page.

Removing a project's sprint schedule requires:

• Project Administration rights on this project
• The project cannot have any closed backlog items, defects or any epics scheduled in a sprint. To verify this, check the following pages. In both cases, you must customize the page to show the Sprint field and change the filter to Show Closed Items.
• Backlog page in Product Planning
• Epics page in Product Planning - no epics should be associated with the sprint
• The project projects cannot have any closed retrospectives associated with an sprint