Fun little platformer! The art style is very pleasing to look at. The controls are simple and it wasn't too hard to jump to the next platform.
Some things are obvious, like the key and lock and the 3 hearts above the player, but I didn't figure out the red balls until I got to the house at the end and it told me that I needed to collect 10 of them.
The spawning and the movement of the red balls and green enemies seemed random so I think that's good for replay value. It makes the game more unpredictable in a good way.
I also liked how the next level was a little more challenging than the last level, but not overwhelmingly so.
Good job!