Could have been a better shot if it had a more suitable background also use of the flash has caused harsh lighting both on the subject and the white wall in the background. Try shooting during the day to make most use of natural light and stand the subject away from the background to make it easier to blur the background out.
Could have been a wonderful shot if you had payed more attention to the background... This challenge was about negative space, meaning everything that is not the subject. Take the same child and have a solid green lawn or even a solid white wall behind him and you have a MUCH stronger image.
Also, I've found when shooting children, especially young children, it often helps to be on the floor looking up at them or maybe eye to eye...
You have a great model to work with :) a little work on the technical aspects and you'll be all set :)