Here are my suggestions:
1) We might need checkpoints starting around 1-4. That green walker is a pain, both to get to and fight. I mostly run from it and try to get through the blocked doorway rather than fighting it. Also, I would look at the passage right after. I feel like the mines are too close together (but I could also just be really bad at the game.)
2) Maybe more clear means to heal the character as well. Keep up the cool pixel art, though. I love the feel of the game, even if I get frustrated. NES Castlevania type vibes.
3) I'd also suggest having an "up" and "down" control for attacking, whether with the gun or the blade. Also, "up" would possibly help with the green walker and floating eyes.
Otherwise, I like the design and feel of the game, and it could be potentially played in Opera GX browser games, as I'm pretty sure you can set age restrictions on it as well.