Or bring it in closer; I routinely work with lighting only a foot or two from the subject. The closer the light, the [apparently] larger it is, and thus the softer. It looks to me like you're lighting the back wall (either intenionally or accidentally) and it's close enough in intensity and luminosity that it's competing with the child. Try going for a background 1 - 1.5 stops under as in this example:
Is a simple, three light set-up. Key is a speedlight high, camera left firing into 30" Ezybox; a kicker high and right for a hairlight (not correctly placed in this image, but you get the idea) and a third speedlight with a blue gel firing at low power on to the background.