This is pretty easily done with a layer with a simple linear gradient, from black to transparent. Alone, that looks like:

(In the above, the background is rendered as white, but in the actual layer it's transparent). Change the opacity up and down to taste — as you can see from the top of the image there not being black, I've put it down to about 15%. Combined with your original, that gives us:

and then, since that makes the overall image dark, a simple curves adjustment to bring up the midtones overall:

You can, of course, adjust the strength of either of these to taste. In this version, the brightness is generally even overall, but you could instead go more dramatic:

This is a pretty simplistic technique, and one thing to watch out for is for dark shades going to flat gray when they shouldn't — here, particularly apparent in the people on the walkway.

An alternate approach is start with bringing up the dark foreground. I used this Shadows & Highlights Gimp plugin, but you could use the (better, I have to say) Photoshop tool equivalently. I used a layer mask with a gradient (basically the inverse of the above) to apply the correction only to the lower half of the image. (Were I spending a little more time on this, I'd also mask out the middle-left tree and treat it separately.)

Then, I used the first gradient technique to darken the top, here choosing "soft light" as the blend mode for variety:

Again, there a lot of knobs and dials in this process where you can change things to meet the vision you want.

I really like the trees and shadows on the far shore — without them, the image is "girl by edge of indeterminate body of water", while with them, it is "pastoral landscape with girl by pond". My initial wish is that there weren't so many people walking around in the top left (and might go so far as to paint them out, which wouldn't actually be that hard), but on reflection, they are really part of the overall story of the park.

For what it's worth, for that reason, I most like the version one above, with the foreground brightened and the background also left bright. It's true that the walkway attracts attention, but that's not necessarily bad in the overall context — my eye jumps there, and then wanders down the diagonal to the girl, then up to the far shore, and then back to figure out what's going on with the ducks... taking in the whole image piece by piece rather than just immediately dismissing the whole thing as a straightforward snapshot.

In fact, with that in mind, I'd almost wish for more of empty space to the foreground bottom left, showing more of the path, to give a juxtaposition of the kid's quiet, empty corner vs. that busy area across the pond. That's a nice, successful "story" photograph. At least, that's what my mind is filling in — maybe the quiet corner is a busy parking lot in reality. :)