All photos added to Lightroom on the iPad are uploaded to the cloud. Sometimes this takes awhile depending on the speed of your network. You can check the uploading process and the sync status by tapping on the cloud in the upper right corner of Lightroom. An even better way to confirm everything has uploaded, is to visit your lightroom cloud at lightroom.adobe.com. Everything you see here has successfully uploaded to the cloud.
Clearing the cache removes temporary local storage of your images and is safe to do because your images are in the cloud.