My 2 Cdn cents worth:
Why bother with another camera since you already have the 5D and a preferred zoom lens?
To help you carry it all day you might consider an UpStrap ( http://www.upstrap-pro.com
). With my film Leicas I was most happy with the thin leather straps wrapped around my wrist, the Canon strap was basically just a piece of advertising always slipping off my shoulder. The UpStrap sure changed all of that -- the camera hangs securely of my shoulder and is but invisible until I bring it up.
I have a 5D2 (got tired of cleaning the sensor, plus the 'upgrade' deal was too good to miss). I travel with it and a 24-105 as well as with tiny 20 and 50 Voigtlander lenses. Both the VC lenses fit in vest or jacket pockets -- neither are auto focus so that may be an issue).
Check your metadata and see what focal length you use most often and pick a fixed focal length lens accordingly.
For backup I just have a Canon G10. Lots of other choices there.
A Leica M9 would be ideal for me since I have lenses but $7k is a lot. But for you, unless you like and know how to work with a rangefinder, testing that on an important trip may not be the best thing to do.
Plus, you have a son: load him up with the gear -- what are kids for anyway!
Enjoy your trip,