From the Library of Congress. The caption is simply "Unidentified soldier in Union first lieutenant's uniform and child in Zouave uniform with model 1860 cavalry saber"
My guess is simply 1861 or 1862 eastern officer. I can't imagine too many "family" portraits like this after the bloodshed of the Peninsula Campaign and Shiloh. Makes me wonder if dad was trying to get his picture taken before leaving and the kid would not stop begging to be in the picture. Then, being a good dad, the kid gets into the picture with a chair and a sword. Is mom somewhere off-camera? Or is the picture a surprise for when he leaves?
Sometimes the photos leave you wanting to know more.