Intro:

I think its a relatively straight forward build. I did the multi-colour TopCover by using the Pause at layer function of Cura. 

There are two Mechanism files, one soft and the other stiff, this refers to the printed in place spring that clicks when you insert the key. 

I also kept the TopPlate separate for two reasons:

1. The mechanism should be printed with the spring on the top, which would result in the top face you see being a first layer, not desirable.
2. You can easily make your own top plate and change the look entirely. 

Print Instructions:

Pay attention to print orientation:

1. TopCover : Print with the flat surface to the build plate
2. ArmMount : Print with the flat surface to the build plate (.stl orientation is flipped)
3. Mechanism  : Print with the flat surface to the build plate (.stl orientation is flipped)
4. Key : Print with the flat surface to the build plate

Settings used and came out good:

1. 15% infill, zigzag
2. 2 perimiters
3. 4 top layers, 5 bottom (can reduce bottom layers)
4. PLA

To create the colour contrasts on TopCover:

See these nice instructions by Prusa: 

Or by Bryan Vines: 

Build Instructions:

Once the parts are printed, wiggle the spring mentioned above loose (it will slightly stick to the layer below):

Next, use some superglue to glue the Mechanism and the ArmMount parts together, add glue to the following surfaces (do not add to the spring!!):

Place the Mechanism on top of the ArmMount and allow the glue to cure:

Add glue to the flat surface of the Mechanism, and add the TopCover part on top of it, allowing the glue to cure:

Done (for the printed parts at least)! The key should slide in and out with a satisfying clicking action:

To attach the morpher to a child, I glued some elastic band to the ArmMount part:

Warnings:

The Morphing process can cause permanent awesomeness. Also, hold on tight!