Cute and well-polished game, great job!
I kinda wish that the player would have to actually think and strategize on where to use the keys. Right now (except for lvl 2, I believe), you either use the key right away the moment you got it, or complete a short platforming section and use the key when you have nowhere else to go. This, I think, makes the level design quite linear despite having multiple dimensions. Effectively, nothing in the moment-to-moment gameplay of most of the levels would change if you removed the keys and just stitched the dimensions together into one left-to-right platforming level.
However, the game is still very fun and has a lot of potential if you go a little deeper with the main mechanics. Keep it up!