What about introducing a photo near the beginning which has all the dogs together on it maybe at camp with a flag the same colour as their bandanas will be. If you let me have a few photos of your dogs I could use Gimp to make them into one snapshot and give them a really adventurous backdrop. The dogs could obviously love the photo but it makes them a bit sad. Tricks like paw on the photo, picking the photo up and carrying it over to the bed, nose touch the photo, wave at the photo.
Then the bandanas and letters come - not every dog has to be shown getting the letter and opening the letter, and not every dog has to be shown reading the letter, we just need the impression that all of them got it and read it.
They all wear their bandanas and go out excitedly to do stuff and they could do similar things so the things they do link them even though they are far apart. For example Zac climbs up a tree, fade to Missy coming down off a tree stump and spinning, fade to Chloe spinning and then crawling into long grass, fade to Piper crawling out of long grass and jumping over a log, fade to Gus jumping over a log... They are not the right tricks obviously but you see what I mean. For stitching together this part we should try to take as many shots of the tricks as possible so we have a good chance of being able to fade them effectively.
Then a good finish might be the dogs going to sleep with their bandanas, photo and letter (plus whatever other doggy "treasures" you want to include). Final shot zooms in on the letter for the first time and we can read the words "see you all next time"
I suppose we could even do a sort of romance because we could fake two dogs living far apart when actually they live near each other and then we could fake a reunion. And then all the other dogs get letters about it and celebrate in their various ways.
In fact unless someone replies quickly I think I'll be visualising a feature film!
Kassidy if you like this idea I will help pull it together (if you want me to) as I know it will be a lot of work to make it work.