@johanna-kim Awesome work, I wouldn't like to comment on anything that would require an whole redo if this has been done in traditional materials, maybe just some thoughts for another time..... I really do like this piece, but one thing that stands out to me is that my eyes are drawn to the girl in the middle due to her pose and the composition of the painting.
I love the washed out colors of the background and the props are awesome as is the patterns of the stone. Although I did notice that the tone of the black swimming cap is quite strong against those colors, which pulls my eyes to that side somewhat (this could be fixed in photoshop if you find it to be an issue yourself). You have given the girl in red the most contrast though and the dots on the cap are a great idea to make her the focal point, so I wouldn't say what you have done is wrong tbh.
Also i'm guess I would need to understand the rest of the book, but is the octopus a ghost/spirit of some kind? I ask due to his transparency/translucency (or maybe you haven't finished painting him yet). In either case some translucency would be cool although the background within it should be distorted if so. Also not super important, but the life ring at the top left of the screen gives more weight to that corner than is needed.
Anyway great piece, and I'm just trying to find whatever issues I see to be of help, but even if nothing was altered I would still say it looks great :D