Cute game! I love the fact that you made this with your 10 year old! It took me a minute to realize I had to use the arrow keys to select my character, I was trying A/D since I typically use WASD for movement controls. Sometimes the level 2 enemy would just spawn and kill me instantly, so maybe a bit of rebalancing on where it spawns would be nice for gameplay. I also thought the double jump for the square character was a bit tough to work with. Overall nice entry and congrats!
P.S I noticed the smiley face on one of the platforms in level 2 and it made me smile too