If you want the dates in your parent project to reflect the planned completion based on the cross-project predecessors, you can setup the dependencies as defined in the example below. If timelines change in the child projects, you have to manually recalculate the timeline of the parent project (or wait for it to happen automatically overnight).
Parent Project
Task 1 (Create cross project predecessor to Child Project 1, last task in project, Finish-Finish)
Task 2 (Create cross project predecessor to Child Project 2, last task in project, Finish-Finish)
Task 3 (Create cross project predecessor to Child Project 3, last task in project, Finish-Finish)