btw, my 6 year old played this and loved it... his favorite was the last level where you do all the simultaneous jumping... definitely do more of that early on.. introduce the mechanic slow and let people really play with controlling both chars at once..
referencing that Miyamato video.. where he says something like: we want Mario to be big, that's where it's fun.. we back track from there.. how do we get Mario big? and that's how you figure out your "pacing".
So I think some of your levels are really challenging early on. Where that last level my 6 year old could do easily.. even if he failed, it was so fun he wanted to try again. Whereas some of the other early levels were confusing and if he wasn't an avid gamer would have given up.
maybe do early levels where the chars are on the same side.. then slowly as the difficulty increases separate the two...(like in the first few levels)