Black levels and colour cast, like Ted suggested, are the most important.
There is a haze to things further away from the camera, and due to how different wavelengths of light are absorbed by the atmosphere object in the distance usually have a blue tint to them.
When it comes to the black levels, the darkest shadows in your foreground should be the same level and tint as the darkest shadows at a similar distance in your background. As things get further from the camera the diffusion caused by the atmosphere means that the blacks don’t look as dark. You can see this in your plate. The trees nearest the camera have darker shadows, and therefore more contrast, than trees further away. You need to work out where in depth your kid should be and try to match those levels.
Another good trick to quickly make something sit in with a background is to add some light wrap from the background over the foreground. Take the alpha of your forground and blur it. Then take another copy of your alpha and use that to mask the blurred version. This will give you a soft feather inside the shape of the foreground. Then take a copy of the background, blur that a bit, mask it with your feathered alpha and put this over your foreground. Adjust the blur of the first copy of the alpha to control how far this wraps around the foreground and the opacity to control how strong it is. In this case you won’t need too much, but even a tiny amount helps.
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