Shouldn't be too difficult.
Can you get two of your flash heads on the floor above the staircase, pointed to bounce off the ceiling? If you can, and the ceiling isn't too dark, that should produce enough bounce light to create a natural-looking overhead light.
Then place your other two flash heads, one either side of the camera and arrange your people on the staircase. It would be a good idea to get the very youngest and oldest at the foot of the stairs, sitting, and other people can just be on the stairs. Make sure that you check with them that they can all see YOU clearly - if they can't then your camera won't see them.
Don't waste your time with softboxes, umbrellas or anything else. They won't help at the distances involved and will waste a lot of light.
The name of the game here is to get fairly balanced, adequate lighting, not 'good' lighting.
Try to get an aperture of somewhere around f/8, to provide enough depth of field (that's just a guess, check for yourself) Set the lights on the ground floor to full power and the ones on the landing to whatever they need to be to produce decent results. Use the lowest ISO setting you can, increasing it as necessary to achieve your f/8 setting.
Hope this helps
