Ok. That was the most awesome introduction to a clopfiction I've ever read. Nice, silly and quick, but still somewhat realistic.

treasures guarded by the multi-millennium old guardians of Equestria

Hmm Celestia and Luna are canonically just a bit older than eleven hundred years... with high probability less than twenty hundred.
On top of that the plural of millennium is millennia. You should replace that with millennia.

keys, accept for one,

except

each the size of a small watermelon.

Uh... even a SMALL watermelon is BIGGER than Twilight's head... Even on Celestia they are going to be grotesquely oversized. Something a bit smaller? Grapefruit?

She layed on her back

laid [at least that is the common use]

hiding the bolder obstructed

boulder

She spun back around and clapped her hooves in excitement. “I can’t believe I did it… I wonder if Cadence knew about our dance...”

Part of me believes she knew exactly what she was doing...

She gasped, for before her were the most precious treasures of forgotten history. A matching set of Alicorn jewels. Two sets of massive testicles and sheathes, one white, one dark blue. Resting within the box on a soft silky pillow. The base of the organs glowed with powerful portal spells, the cause of their misplacement.

Portal spells? This means the organs are still connected?
Better don't touch them if you don't want to get in trouble.
Or do touch them, might have interesting results.

The guards sighed, watching the mad princess run off to who knows where, they picked the doors up and used the repair spells they had been taught when given the honor of being the personal guards of the moon princess when she slept.

That wasn't neither the first nor the last time they had used that spell, right?

Plus it would be a shame to come all this way with her gifts for the princesses and somehow lost them on the way.

Yeah, that would be fun (?) explaining it to them.

Celestia then took care in removing her sister’s bed covers, revealing the majestic mare of the night, curled up with a teddy bear and sucking on her hoof.

Luna: "Tell anyone and I'll throw you into an black hole."

Twi wouldn't be embarrassed about Ladybugs Awake, she's done it in public and in front of her friends many times, including the first time when we first met "Princess Mi Amore Cadenza"