Hover over the child site in question on your left sites list and click the View backups link. Or go to Protect -> Backups. Here select the child site and backup which you wanted to restore.
Now click the Restore Child site button. IWP will restore the Database and uploads folder on your server.
Note: Only the Phoenix backup method will support the Child site restores. So please make sure that you are selecting the Phoenix method while backing up your parent site.