Questionable answer:
Buy a second-hand Metz 60 CT1, that's a GN of 60 in metres, 100 ISO, or 120 in metres, 400 ISO. Set if fixed on a background wall; They are not expensive as they don't have any way to be "dedicated" to a camera (early models).
Use the Rolleiflex with its own flash to lit the baby as usual;
Find a way to synchronize your flash with the other one with photocells.

Stupid answer:
Buy a couple of big and powerful halogen lamps (the normal one, for your living room) or change the room bulbs with more powerful ones; Increase ambient light, that is. If you use B&W, anything will work.
Use 135;
Use 35mm;
Use f/5.6 or so. The background should be subdued but not sadly black. The DoF should be enough for "portrait" (supposing you want the take the entire figure, the environment, the toys, part of the house etc).

Blasphemous answer:
Use digital to practice. Revert to your Rolleiflex when you found the right setup.