I like the mixture of media and the idea, the stop motion in particular is really well done. If you wanted to aim for a slightly more 'child-like drawing' feel for backgrounds, a bit more pastel colors would've also furthered that (rather than like max saturation blue for the sky). An overlay of a paper texture can also help with this and sometimes works nicely.
In general, reducing the saturation a little bit could help with color balance overall--but all said and done, the mixed forms of media still flow together nicely.
The big thing I feel is holding this back however: the sound balance is pretty rough. The music riffs are a lot louder than the dialog, so loud that if you have the volume up to hear the girl, the music cuts in painfully.