Agreed with ShinigamiArtist. The boy and the girl seem to be the same sprite just mirrored as well - with admittedly slight alterations like the hair and shading. The stars and even their patterns were also taken from your other piece. There's just very little original content here.
At this point I don't understand why you didn't just add fireworks (your own style) to the dragon piece and be done with it. If the answer is that you developed the dragon piece before the challenge, then... 
Agreed that it's a nice piece on its own, but as a challenge entry, it seems a little questionable. Sorry.
It's nice, but I noticed you used pieces from past artwork and used another entry's firework style.
I was started to draw past art for challenge, but i didn't see the size limit. I got carried away and wanted to make a separate work