This is the first thing I’ve made in Blender that I’ve actually finished. It’s also the first time I’ve ever used the compositor, so I played around with that a lot and I might have done more harm than good because of that. And I’ve never done grass before, so this has a lot of firsts for me. 
So because of that, I want to know what I did right and what I could have done better. Thank you for any advice.
Way better than my first (and current!) renders 
But for critiques,
It kind of looks like a cartoon, its not realistic.
The branches don’t look like they’re attached.
The rope or string to climb u on looks… like string.
Really nice scene though, it looks like it should be on a book for kids or something.
I agree that this is good and I like it. It does need work though. I think that overall the scene feels flat both in lighting and scenery. Perhaps you could have more dramatic lighting cast from the moon to the objects and maybe a few distant hills. Also, why is the moon the only thing with a black outline?
Other than those things great job
It looks nice! I agree with the previous comments, especially with what Billt Joe said about the lighting. The key to lighting a night scene isn’t darker lights, but higher contrast. In other words, have some strong highlights and some dark shadows. Great job so far, though 
Yeah, I had a hard time trying to get the lighting right. I didn’t want it to be too bright because it’s at night, and if it was too dark then everything sort of blended together. I tried to fix it in the compositor and i guess that’s why it looks like a cartoon.
I hadn’t noticed the black line around the moon, that must have happened when I added the glow around it. It’s not supposed to be there.
-Your branches seem a bit non-realistic. How did you make this tree? Have you tried the tree-generator add-on that comes with Blender? It works like a champ. It can also make your leaves more realistic.
-Your tree house plates have a rough looking texture, though they are completely flat from what we can see. Use another texture that does not resemble rough wood, or activate normal mapping (by checking the checkbox next to the ‘Normal’ slider value in the ‘Influence’ panel of your texture) to make it more realistic. The support beams below are very good.
-The lowest branch, which is on the right side, Has a different color than the tree itself. It seems lighter, creating a very unrealistic effect. make sure the tree and branches have the same colour. If you want the branches to have a slight color gradient, use a blend texture.
-Your tree texture seems flat. Enable Normal mapping, or increase the normal mapping value if you already have.
-The moon is behind the leaves, leaving an open gap to the left of the tree. Why not add some branches and put a swing in your scene? Very cliché, but timeless and a good addition to a scene like this
-What is the use of the rope’s position? If someone tries to climb it, the branch it’s hung over will likely snap, injuring the person. If you would like to add some use to the rope, edit it’s pose or perhaps, hang a tire on it for a swing.
-The moon is dark. You need high contrast for night scenes. Use the compositor to make the moon brighter and the other object darker (you can do this easily through the ‘Brightness/Contrast’ or ‘Gamma’ nodes). Also, the scene look as if some form of global illumination was applied. Use a whitey sun lamp to act as the moon, and use Area lamps as fill lights. Do not use any yellow or red colours, as they simulate daylight.
-To the bottom left, there’s a slightly odd transition in the grass. It seems as if there’s another hill behind this one with the tree on it. If it is another hill, position it otherwise so the horizons of both hills don’t look like one line, and if it’s not another hill, fiddle with your grass.
That’s all I can find. Hope it helps :)!
Happy Blending!