Hi Christine!
Thank you for your question, I'm happy to help you!
Here's the link for the pre-arrival PhotoPass CD. The terms and conditions are on the bottom of the page, and you can order the pre-arrival savings and it acts as a credit. Once you have returned from your trip, you can add the PhotoPass CD to your order and the credit would then be applied. Telling you when to order is difficult, because the pre-arrival savings may or may not be around for a while, and also there may or may not be similar offers between now and your next trip. If it's really important to you to get it at this rate, then purchasing it sooner would be best.
PhotoPass photographers are actually pretty easy to find, but to add a special touch to your PhotoPass photos, ask your PhotoPass photographer about super imposed images of characters and such. These are available at some of the locations, but they may not always offer them to you. If you'd like, there's a great guide that goes over these locations and the various shots available, as well as other PhotoPass info, over at allears.net (search "photopass" once there). You'll even find a pdf to print out if you'd like to keep it on hand for your trip.
Of course, you're welcome to ask more questions! I'd be happy to lend a hand in planning your family's magical vacation!