Take them when it is convenient and don't worry if you don't have much time.
I snapped these of my good friend and her daughters in under 10 minutes before they were off to a wedding. I love that in a few minutes, photos that capture the excitement and anticipation of a fun evening are forever recorded. Life holds so many beautiful moments.
Editing tips: The photo above, I brighten and increased the contrast in Photoshop, then ran the Vintage Matte Free Action from the Coffee Shop Blog. Totally my favorite photo.