Tom,
Photography about the human condition means so much to us. These pictures about children have our hopes, love and fears all bundled together, jostling for our attention. So there's a lot to think about.
Obviously you are right as usual in the value of unposed work, but I'd imagined, in the O.P. picture, the child was unaware of the picture being taken. Memento's are indeed family treasures and all these pictures will be important, far into the future, to others looking back at how we raised our kids in these "olden" days.
Of course, the picture by the beach is immediately seductive and allows adults to muse. But, I must admit the same applies to your picture.
Now if your picture was in a national gallery, (and it's that worthy), folk could and would write pages about it's significance.
Asher