I grew up in Puerto Rico with this Spanish tradition and have many sweet memories of Los Reyes Magos (The Wise Men) so it's neat to see it being passed on to Grace and Hudson. On the eve (January 5) of the day the Wise Men reached Baby Jesus in the manger (January 6 - Epiphany), little children cut grass stuff it in shoe boxes and place it under their beds. The grass is a snack for the camels of the Wise Men who "stop by" the homes of sweet little children in their slumber as they make their way to meet and bring gifts for the Christ child.
The Reyes only come if the children have been good all year and if they are awake they bypass their house. Grandparents, aunts and uncles ask children to put shoe boxes of grass under their bed too - just in case.
I fondly remember the year I wised up (no pun intended) and realized my grandparent's king size bed was big enough to stick a shoebox under EACH my Tata and Abuelito's side so that the Reyes would bring me double the gifts!
In the morning when the children wake, the Reyes have left them little toys and trinkets to remember the gifts they brought Baby Jesus. The camels are a bit messy and often leave footprints and a bit of grass scattered every which way. Sometimes they even leave a letter reminding good boys and girls of important things...like why it's important to not scream and be good at the dentist...or why using the potty ALL the time is THE cool thing for big kids to do :) In many Spanish speaking countries, the 3 Wise Men are a much bigger deal than Santa. Many have real-life manger scenes complete with not only your traditional Joseph, Mary, Baby Jesus, but live camels, donkeys, and sheep. When I was little I remember going to El Morro every year a historical landmark in Old San Juan to see the manger scene. I was always amazed by the enormity of the camels next to the tiny baby Jesus. Such sweet memories!