My girl is almost 7 months old and barely recognizes me without a black square in front of my face. I think I have shot 99.9% of the 99000 photos of her with my 50D and a 50mm 1.4. You could do the same with the 1.8.
A good investment, imho, would be a lightstand, shootthrough umbrella, and a cheap trigger set. Then you won't be stuck trying to catch window light, AND if you do have good window light, you can use your flash and umbrella setup for a little extra fill.