My daughter wanted me to make a fantastical heart gears for her, but it was kind of big for her to handle. So I took inspiration from the keyed heart gears pendant/keychain to make it smaller and functional. I tried taking it down to 50% of the size of the original, but some of the details were getting too small to print reliably, so I settled on 60% of the original size.
I added a loop to the big piece and made a hole through the bottom for a key. The heart shaped key (of course) connects to the center so that you can hold on to it while turning the loop. This made it very easy for my seven year old to work it. My three year old neighbor even turned it very easily (now I have to make her one too).
Here's a video of my daughter turning it:
I printed mine at .2mm layers with two shells and 10% fill. I have a fan blowing on the print which helps with the overhangs. I ended up with a few little strings that cleaned up easily.
Before assembling, I sprayed CRC Heavy Duty Silicone spray on all contact parts and let them dry (This stuff works great for lubricating moving plastic parts). You need to let the spray dry. When it is wet, it actually makes the parts stick more. After a few hours of drying, you will be amazed at how well the things turn.
To get the tiny pins to work, I used a pair of needle nose pliers to squeeze them in the middle a few times to make them more pliable and skinny.
Next, just follow Emmett's assembly instructions here: http://www.thingiverse.com/thing:243278