First I want to say that this is an adorable image, so your idea is creative. However, there are some misjudgment in the way it was executed.

There's an art element having to do with space called atmospheric perspective where things in the foreground are larger with more detail then the objects in the background, getting smaller with less detail as they move through the middle ground.

I look at your butterflies and they appear to be in the same place in the picture plane because they are the same brightness and each have the same detail. Yet you made some smaller than the others so I think you know that part about perspective, but not the "detail getting fuzzier" part. The same holds true with the balloons, they shouldn't be as bright as they are in the sky.

Also, shadow is lacking below the figure. I'm trying to find the strongest light source to determine which side the shadow should be and in the sky it looks like the shadow should be on the left. I would make the light on the right side of her body a little brighter, the dark side a little bolder, and create a cast shadow on the left hand side of the grass, other wise she doesn't look "grounded".

I think the best part about this manip is the fact that I could tell what it was about before I even read the title. The theme and meaning is obvious and hits you immediately leaving no room for question.
You achieved the goal of nostalgia in this piece, reminding us of our younger, and, perhaps, more innocent days.
The only critique I have for this piece are that the bubbles seem out of place. To me they are a bit to much and their surface reflection doesn't match the overcast sky.
Speaking of the sky, I love how the clouds give a sense of something more sinister of a hint at the loss of innocence. The lightning in the background is perfectly subtle.
Very nicely done, Plea.