I am looking for advice on which lens to use for school portraits. I have a Canon 7D which will remain stationary on a tripod for the duration of the shoot. I'm using Alien B800's for lighting, and only one backdrop for the entire shoot. My subjects will be between the ages of 3 months - 6yrs, so using a prime lens is out of the question (for subjects in this age group, I'll constantly have to adjust to their movements). Finally, my camera will be positioned about 3.5' from the posing table, and I would like to find a lens that would allow for a headshot, 3/4 length & full body shot without distorting facial features, while remaining versatile enough to possibly capture a much wider class photo. I would sincerely appreciate any advice 

