Other drivers will become desensitised to the image, learn to ignore it, and possibly then ignore a real child in the future.
It's hard to express how shockingly bad this idea is.
Before the image itself, there is "a School Zone sign, crosswalk, an extended curb and a sign by Preventable that reads: You’re probably not expecting kids to run out on the road". It's hard to imagine that anyone would be going fast enough to cause any damage.
Furthermore the image was there only for a week. I somehow doubt drivers would suddenly start to ignore children, after they saw fake image of a child five times.