Probably the easiest way is to move all your photos to a computer, edit there, and then move back to your iPads via Dropbox, email or iCloud (AirDrop in the future?). We have 2 iPhones, 2 iPads and MacBook Pro laptop when we travel. We rarely use our iPads to take photos but do use our iPhones extensively.
There is a way to move photos from iPhones to an iPad via Apple's $29 30-pin Camera Connection kit via plugging in with the iPhone's USB cord or SD cards from other cameras. If you have the new connector iPads then you also need the adapter. I don't see where Apple has a new adapter camera connection kit.
Camera Connection KitLightning to 30-Pin Adapter if you need itWe are moving away from physically transferring between iPhones and iPads to simply using Photostream on iCloud. Take a photo on your iPhone and it automatically transfers to the iPad and computer whenever you have them connected to wifi (not cellular). The trick is all the devices have to be on the same iCloud account. That's not practical for us since an iCloud account also syncs calendars, email, contacts, notes, etc. It is fine for me managing my own photos but not my wife's.
I put all the photos from all the devices onto my MacBook Pro directly into iPhoto via Photostream from my iPhone or Camera Connection Kit from my wife's iPhone or Camera Connection Kit for our other digital cameras. There I can edit and organize and create slideshows amongst other things.
Photostream via iCloud is cool. It streams everything to my computer into iPhoto and keeps the last 1,000 photos active and viewable on all my devices including Apple TV on my 47" TV at home.
Of course there are other ways. You can send all your photos from all devices to one account online such as Photobucket, Flickr, etc., organize, and create slideshows there as well.