Look at the back of the dog, the light is spilling on it and the head and the little girl's head. You are basically dealing with a giant light source and have to watch that it doesn't bounce onto your subjects.
Its the hardest part of getting high key correct, getting the background 100% white, yet not having that light spill onto your subjects
Zack Arias has a great post on this
http://www.zarias.com/?p=71