Have the mum hold the infant, and the dad the toddler child. Seat both parents. Make SURE the babies are positioned so that their heads are HIGH-UP in the photos!!! The mother can cradle the baby across her chest with the baby's feet pointing "in" and toward the middle of the group. The father will help control/position the second child.
For light, a single SB-800 fired off of a wall and ceiling, or a wall/ceiling interior corner, can create a large, soft source of light. To ensure adequate exposure when bouncing a flash, elevate the camera's ISO to 400 or so. You have a modern, d-slr camera and a good quality flash unit. You ought to be able to set up the shooting situation so that the flash head can be turned upward and angled/swivelled around and aimed so that it bounces off of a large painted surface, and casts its light in a simple, soft manner toward a group of two seated parent, each of whom is controlling/posing a small child. Many times, I will set the flash's Zoom head control to around 85mm, to get a good, strong bounce; in very,very small rooms, I will sometimes go to opposite way, and set the flash zoom control to 24mm. "It all depends". If the flash beam must go a long ways in order to make the first wall or ceiling contact, zooming the head to 85 to 105mm is good practic.
This ought not be a difficult shooting scenario...do some tests shots with the flash actually aimed different ways, so that you get a good bounce. And as I said, make SURE to elevate the ISO to 400, at minimum!!!!!