Upload & Sell: On
To take a few steps back, you’ve specifically asked about props, vendors and so on so what I’m about to say may be stuff you’re well familiar with and of no use. I’ll offer it anyway in case this is new territory for you.
There are a lot of ways to approach this but rather than props, etc, I suggest you initially focus on the fact that you’re planning on shooting a lot of portraits in a short amount of time. You’re going to face stress and challenges on several fronts including the technical aspects; posing; and the biggest headache, the logistics of coordinating people in and out. A few thoughts:
You could use a well organized assistant (or two) to handle scheduling, organizing people in queue, handling mishaps along the way, etc.
Consistent lighting and dialing-in/documenting/repeating your camera settings will be important. All the creative stuff needs to be done during planning, initial set up and test shots. Once families start arriving, your project becomes a matter of production, not art. During “production”, the only creative aspect you should be dealing with is posing (can be a challenge) and even that should be “templatized” for the basic shots.
You’ll need to do practice runs/shoots with a few friends to get an idea how it’s going to play out and how much time it will take. I would personally allot at least half an hour per family. You may find you nail some portraits in a few minutes (depending upon subjects and serendipity) but it could easily take much more time.
Next, what are your plans regarding lighting? Sculpting the light is critical but repeatability is the bigger issue. You can do this with ambient light… or do you plan on using flashes/strobes? If the latter, do you have that gear already and if not, what’s the budget?