I do like the overall idea. But these are my suggestions to make it a little better.
First, you have to be careful when choosing stock, especially your model stock. Despite how cute the child is, she is making a face as if she is about to cry. If I noticed it, others probably did as well.
Also, her dress lost most of the shadows, which makes her look like a flat shape.
Next, both she and the horse still look like cutouts. The edges need to be soften and blend more into the background so that they look like they're a part of the image, not separate from it.
The sky is too flat. You didn't give credit to any texture used, but that's what it looks like. Whatever is the reason, a new sky with some clouds would help.
Lastly, the butterflies. Many deviants make this same error: Making the butteries too large, and making them all fly in the same place in different places the atmosphere. And you have each butterfly in the same shape, instead of choosing them in different positions. Like a couple with wings fully open, a few middle close, etc. That also would be more realistic.
If you want to know what I mean about how to put butterflies into a deviation, and more about how to get space and depth, see my here:
What it is missing is the element of light and shadow. You may get the feeling that it would "appear" you cut and pasted each element of the piece onto the canvas. That is how it will feel if you don't implement natural light and shadow effects.
For example, where is the sun in this piece? Even if you don't depict it physically in the piece, it lift should still be seen in felt. The little girl is in a bright sunny flower filled field, if the sun is off in the right hand portion off canvas, her face, the carousel horse face, the pole, hands, dress, legs, butterfly wings, all should lit with light, then shadowed naturally.
Light and shadow is where I struggle in the manip process as well, but it will make your pieces feel real and natural (no matter the subject matter) when implemented in your pieces.