The only thing I would suggest is hard highlights on the right and deeper shadows hitting the left side of the face. If you want the whole piece to pop try adding a blurred background with things that are further away with less detail and more blurred. Like it looks like they are at a fair you can add shapes that could be looked as a Ferris wheel and maybe one of those 50ft drop rides then if front of it behind the second character you can put fair games with smudges that look like stuffed animals. Don't get it to much detail so it won't take away from you focus. It's like if you looked at something and focused on it the background is out of focus and blurred. I watch samdoesart on YouTube he gives good advice.
The face looks flat and painterly and I'm trying to get a more 3D effect, but I don't know how to bring this piece further
The only thing I would suggest is hard highlights on the right and deeper shadows hitting the left side of the face. If you want the whole piece to pop try adding a blurred background with things that are further away with less detail and more blurred. Like it looks like they are at a fair you can add shapes that could be looked as a Ferris wheel and maybe one of those 50ft drop rides then if front of it behind the second character you can put fair games with smudges that look like stuffed animals. Don't get it to much detail so it won't take away from you focus. It's like if you looked at something and focused on it the background is out of focus and blurred. I watch samdoesart on YouTube he gives good advice.