lol, I actually made firework rockets once in a game of team lucky islands.
Very sadly, they didn't work at all D:
Theres a Map called "Oasis" and on it there is sugar cane to make paper, and you can kill witches to get gunpowder, then combine them to make rockets. Although it was a great adventure creating them and they are cool just to see In game, they sadly don't work, it just plays the clicking animation and nothing happens. I assume this is because of the older version of Minecraft that Cubecraft is run in, this version also breaks a couple other things, for example you can't create observers or boats in game.
And as for the kit... as someone who abuses elytra a ton I would love to see it as a kit but, it would be hella overpowered even without the fireworks (if they were to work that is).