When they are small, it's easy. I'd get a blanket or fabric or something (I'd go for simple or no patterns) and lay them out in the brightest part of the day in full shade. I used my 50 f1.8 on a Canon T2i and snapped away. The lighting makes their skin look all glowy. These are a few of my daughter at about 1 month and 7 months old.
Another nice option is sunset light.
I have no signature.