Depends on your budget, and which digital body you're using. On a crop factor body, a 35mm lens will be closer to a 50mm focal length equivalent. I've been using a 35mm lens on a M8.2, and I really like the results. On a full-frame body, I still think a 35mm lens will work well since you'll need a slightly wider lens for your family portrait as well.
If budget is not a concern, consider the 35mm f/1.4 ASPH. If budget is tight, get the Leica 35mm f/2.5 Summarit which delivers amazing bang for the buck, as well as a compact size.
This following image is shot with a 35mm f/1.4 ASPH on a Leica M8.2. You didn't mention your son's age, but shooting a active toddler with a rangefinder is really a challenging exercise!