i really like this concept. my angle would have been further back to capture a bit more of the landscape. that dog is really key and it's great that it's in the picture. perhaps this was the perfect moment, but i see one coming that if the kids stayed by the mailbox, the dog would have gotten there and been looking up as the looked through the mail.
For me Street Photography is more at home in an urban setting. rural settings don't work. Although a nice shot i would have scored this better in a different challenge.
I don’t have much knowledge on street photography but my thoughts on you shot are as follows: This is brilliant. Very different from the other shots but I really love it. Look at that little dog making sure they are ok as well. Excellent. 10 from me.