Dan,
For me I have found one umbrella the easiest "no fuss" method: I have a little (small) umbrella about 900mm dia. and I can rig a 580 on the end and hold it in one hand using an off-camera cord. Very young babies don't move much. One umbrella for Soft Fill or a Kicker when using Available Light as Key in outdoor Soft Sunlight harvests good results, or, alternatively, one umbrella inside creates a nice but simple "studio" effect.
BTW I meant to mention earlier: the macro/montage idea is a great idea, Kay.
All babies are so cute!
WW