OP

The opening or OP refers to the opening song of each episode.

Events
Lili is the first one seen as the door opens, indicating that the song is sung from her perspective since it's sung by a female vocalist.

The one knocking at the door is Luc with Amala standing next to him.

Sam is actually the one who opens the blue door though, he was inside standing next to Lili.

This means these are the "main four" characters of the show, as indicated by the generic description included in some TV Guides:
 * "Four neighbors living in a building in the big city celebrate each other's cultural celebrations and adventures."

During the "race you to the bottom" line, the order of the main four is: Lili, Sam, Luc and Amala. They are however followed by three more:
 * 1) Eddie
 * 2) Lili's younger sister Lola
 * 3) Sam's elder sister Sara.

During "who's that playing hide and seek" Lili is the one covering her eyes while Sara is climbing.

Instruments
The closed captions describe the tones heard before the first line of the song as "bright, upbeat instrumental"

Lyrics

 * WHO'S THAT KNOCKING AT MY DOOR?
 * I'LL RACE YOU TO THE BOTTOM FLOOR
 * WHO'S THAT PLAYING HIDE AND SEEK?
 * COUNT TO TEN AND DON'T YOU PEEK!
 * LET'S HEAD OUTSIDE AND PLAY
 * CREATE A BRAND-NEW GAME
 * OR SEARCH BENEATH THE STAIRS
 * FOR HIDDEN TREASURE THERE
 * 16 HUDSON: THAT'S WHERE MY FRIENDS ARE WAITING
 * 16 HUDSON: WON'T YOU COME ALONG?
 * 16 HUDSON: WHERE EVERYBODY'S WELCOME
 * 16 HUDSON: THAT'S WHERE I BELONG!