Wow, I had a lot of fun with this one! Love the main menu and music, the overall theme is fun and consistent. The characters are so well made, they really add a lot of charm to the game along with the headbutt mechanic which I had a good chuckle at. Having multiple levels is a really nice touch, as well as the character selection (although I couldn't figure out how to unlock the other characters?)
I feel like gameplay could be improved by having a short delay between headbutts to make hit detection more reliant on timing rather than just mashing the button. The only other issues I had you seem to be aware of so I won't go on about them :)
Overall a really fun, cute game. Well done!