Hi Christian,
I often think of photography as a dance, particularly when it comes to photographing people. If they are moving in a dynamic space, like these children are, then chances are you need to move about with them in an effort to achieve the best composition for the moment. This is not easy and when things are happening fast then you really need to be quick about choosing your moment.
On the whole I like this image. This could be a rather timeless image, but the one thing that is likely to date it is the treatment you have added in postproduction.
If anything I would be inclined to soften the vignette. Within the options for the vignette you should be able to make it less intense while at the same time smoothing gradation of it. When applying a technique you want it to be present, but you do not want it to be noticeable.
The other thing to consider is being a little more consistent with the contrast on those faces. The girl in the right of the frame has good contrast but the boy to the right is quite dark.
Another important point worth noting in photography is that it is often the small details that will let you down. As I look at the photograph I see a line appearing at the back of the left girl’s dress that distracts me. Above her is a bright highlight on the tree that captures the eye but this offers nothing of interest to look at. If you are serious about a photograph you really do need to go and clean up any distractions before you present it. In the old days this meant hours spent with a spotting brush retouching prints; nowadays it means spending time in Photoshop with the Healing Brush or Clone Tool.
Aside from all that though… great effort!
Cheers,
Anthony