I really enjoyed this one! Although the solutions didn't take too long to find, I can definitely see this concept being extended into a really great game. Please let me know if you continue this! I'd really love to see where you take this concept. I do, however, have a few ideas. Absolutely no pressure, though.
First: the controls. While the actual button choice was really intuitive, the movement itself felt a tad stiff, in my opinion. I think that this can largely be boiled down to three very minor things. For one, the player can't jump unless they're on solid ground. While this makes sense in the real world, many platformers actually use a timer-based system, so that if the player leaves the ground there's still an extra fraction of a second where jump inputs are accepted. This makes jumping from one platform to another feel much more fluid, in my opinion. It might be worth experimenting with. I'd also add that perhaps a variable jump height would add to the degree of player control and make everything feel a bit tighter. Furthermore, the amount of time it takes the player to jump up is the same as the amount of time it takes to fall, from what I've seen. While this is absolutely how free fall works in our world, this has really never been true for the classic platformers. In Mario, for example, it actually takes less time for the player to fall than to rise. This definitely adds to the snappiness of the controls, and can really pull things together, if for no other reason than this is what a lot of people are used to.
Next idea: more feedback. As it stands, pushing buttons makes things move, but that's where the player's interaction with the world ends. If you just added some particle effects whenever the player picks up coal and maybe a few sounds for item pickup and movement, I think that this game would be taken up multiple notches. I highly recommend bfxr, by the way. You can get an entire library of custom sounds specifically suited for your game in literally five minutes.
Again, I really enjoyed this! Thank you so much for the game!