Critique my book cover!
Now for the upper half. Hmmm. For starters, both characters look young. Right side, ten years perhaps and left side around twelve years, would be my estimate. Is that intended? Your intro blurb says something about 19 years. There are two issues with the image: On the right side, the blue color seems to indicate hair, as seen by the stray cowlick. You could emphasize this with one or two additional stylized hair strands over the face. Right now it looks a bit like helmet hair. The immediate eye-catcher is the lack of mouths. While there are lips you hint at, the mouths are missing. Maybe it's a stylistic choice since you went for a minimalistic look - but I'd say it's not working out in this case.
Now as for the left side, there is a bit of an issue with the blanket/robe/???. The way it flows to the right and covers that character too is confusing to me, it's perhaps also an unfortunate choice to color it blue - since the hair on the right is blue as well. But... what's actually happening here? It seems to fall down on the left side, like a piece of cloth but then shoots up on the right. Once there, it doesn't adhere to body form or gravity and instead keeps going up.
So in closing: The minimalistic style can work for you - but needs some work itself. My quick and dirty suggestions would be: Add one or two hair strands to the right side. Add mouths (unless intended otherwise). Add minimal one color shading to the blanket. Fix the blanket curvature on the right. If possible, give the skin layer a blue'ish tone, since they're currently at full bright.
Other than that, you're on a good path and if you can get the top layer on the same level as the lower layer, it's a great cover. Keep it up.
There is the very nice choice of colors that do help to hold everything together but it would be better if the other elements of the cover contribute to the whole.
The flipped trees for example is really detailed, rough and busy which it doesn't share with the simple, smooth and clean shapes of the figures/characters. Maybe you could redraw the trees, abstract and simplify them to make them harmonize with figures style.
There is also the font. While it looks nice, it is playful and whimsical which doesn't fit the restrained and controlled feel of the whole image. Maybe find a similarly round and soft font but with more controlled strokes.
But overall, it is not bad. As I said, I like the colors, and the characters, while simplistic, are done with artistic flair.