I have not read all the responses, but I would tell your kids (especially the younger one) when it it tangible. For a 5 year old, that can be very subjective. A neat idea might be do do it about a month ahead of your trip, and make a countdown. There are TONS of ways to make countdown calendars, but a paperchain is the easiest and is very, very easy to see. The shorter the chain gets, the closer you are toward the trip.
For your older one, I think 30 days is a good amount of time as well. That way, he'll (I think you said your older one is a son?) have time to talk to other friends maybe at school about trips they may have taken there and what they liked or didn't like. (rides, food, shows, etc.) I also think giving him a book (we gave DD this one:
http://www.amazon.com/Birnbaums-Disney-World-Birnbaum-Guides/dp/1423169417) would be a good way to get him psyched for the trip. This one was geared to kids, and DD flipped through it quite a bit, even though it was our 2nd trip there.
Here's been our experience:
1st trip - DD was 5 (just turning 6 a few weeks before our trip), and was in kindergarten. she LOVED school, so we told her when we had booked and paid for the trip. She had heard many friends at school talk about going to Disney World, so she had an idea of it being an awesome place. We wanted her to be prepared to miss a week of school (she LOVES school - still does), and in kindergarten would cry if she had to miss school because she just loved it so much! So, in planning, we let her help pick restaurants for ADRs, the kinds of rides she wanted to make sure we went on, etc. I also prepared her for rides that she would NOT be able to go on (not tall enough). She had no idea what to expect as she'd never been there before and she was BLOWN AWAY when we got there. It was a magical time.
2nd trip - DD had just turned 8 (2nd grade) and we tried to do it as a surprise for her. We booked it in Oct, and it was something for me to look forward to as a light at the end of the (Superbowl Prep) tunnel. Of course, she knew we were going on vacation. We had told her we were going to Florida, and she thought maybe we were going to just stay in a hotel near a beach or something. All planning happened at work or late at night, after she went to bed. The night we booked airfare, we sent her to bed early, so DH and I could openly talk about it and book the killer rates before they were sold out! (SW had a crazy sale - $65 each way!!) Anyway - when we got to the airport, we sat her down and had her look in her backpack. Inside were her Mickey Ears and guide book. Her reaction was flat, but she was excited. I think she was more shocked than anything. We realized later that
the anticipation is part of the fun and DH and I have resolved to never surprise her with a trip again, and tell her at least a few days in advance, or have her be part of the planning process, if possible.
Good luck and have a magical first trip!!!