One very basic question that hasn't bee addressed is what kind of mood are you wanting to set? If you're wanting something kind of peaceful you might consider making the banks shallower. Speaking of banks, right now the banks are rather steep considering they have all the vegetation still growing on them. Just completely "what-if" ing, if the child is seated on the left side of the bridge, you might consider actually darkening parts of the background a little more but adding a sunbeam coming through the trees from the upper right corner of the picture to the area where the kid is sitting. It looks like you kind of have that going on now - the right bank appears to be in the shade - but it might help to define the border between shadow and light a little more tightly. Just an idea.

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