While editing a plan in the Aria UI, and you attempt to remove a Child Plan on the Plan Relationships tab: If the Child Plan relationship is mapped to at least one account, you will receive a popup message explaining "This plan is currently in use...", and will not be able to remove the Child Plan on the Plan Relationships page.
If the Child Plan is not mapped to at least one account, and the Child Plan is not mapped to any other Parent Plan, you will receive a warning message stating that the Child Plan will be "orphaned," but you will be allowed to remove the plan.
See Define Plan Relationships Define Plan Relationships for more information.