Tradition matters

"Lavender blue and Rosemary green,
When I am king you shall be queen;
Call up my maids at four o'clock,
Some to the wheel and some to the rock;
Some to make hay and some to shear corn,
And you and I will keep the bed warm"
Lavender blue -Wikipedia

The tradition has it that if you find the little figure of the king in the Roscon de Reyes, you'll be crowned king for one day.

In the extra King Melchor throwing sweets, confetti and streamers to the children during the Parade