Hello there! This is DracoFerret11! You asked me in a review to come check out your story...so here I am! :D So, let's go over things:
Grammar/Spelling: Okay, so, first off I'm going to suggest that you see if you can get a beta reader. If you join the Forums (link on the homepage of HPFF), there are people there who volunteer to read your stories for you to check for grammar, spelling, etc. There were quite a few issues in this chapter, so that's how I propose you fix them. :]
Continuity: Only one issue here--Hugo is a year or two YOUNGER than Rose. So if this was her first year at Hogwarts, he wouldn't be there yet, unless they were twins (which isn't canon, but you're always free to make it AU). ;]
Characterization: Okay! So I like how you wrote for a group of eleven year olds! I absolutely cannot manage writing for children. I mess everything up. :P I liked that you seem to have a pretty clear idea of each of your characters' personalities. That will help you in the long run!
Descriptions: Okay, this is something that I tell literally every author I review for--details, details, details! BUT, know when to put them in! It's important to bring your story to life with little tidbits about how things look, sound, smell, feel, etc., but you don't want to bog them down. I liked that you mentioned that the platform was crowded and smoky, but then you got a little bit too concerned with telling how each of the girls looked. It's important for us to know what they look like, don't get me wrong, but you don't need to tell us all the details right away. Instead, you could have something like, "Dom introduced a girl named Caitlyn. I smiled at her and shook her hand. She smiled back and tossed her black hair over her shoulders, laughing a bit. "Nice to meet you!" she said." Get it? So that it's not like, "This girl looked like this. This girl looked like this. Etc. Etc." :D
Emotions: Okay! Here's something else that you can use to bring your story to life. Try to show how the characters are feeling, especially Rose since she's the MC. Somehow show her excitement. Maybe something about how she is a little nervous to leave her parents. But don't TELL the readers that--SHOW them. Maybe she smiles nervously or wrings her hands together and her palms are sweating. Maybe her heart beats faster as she boards the train for the first time. Etc.
Plot: Okay! Good beginning! Many stories start off like this and it always seems to work out well. :] You definitely have something here that will get readers interested in seeing what happens next. The stormy interaction between Scorpius and Rose will definitely inspire people to want to see how things go between them. ;]
Interactions: So many characters! Okay--I liked that you seem to really grasp the dynamic between the girls. That's good! The interaction between Scorpius and Rose was a little awkward since they're already enemies with no real reason to be. Keep in mind that even Harry and Draco have reached a mutual understanding and have a certain degree of respect at this point. It's Next-Gen, so not all of the old prejudices exist. ;] You don't want things to get cliche!
Style: So, this might actually have to do with pacing too, but I have a pointer for you. :] This chapter seemed quite rushed. There wasn't a real goodbye between Rose and her parents, you didn't take too much time describing her finding a compartment, and there weren't too many transitions that helped things move along. The result was that the chapter seemed to go by a little too quickly. My tip for you is to slow things down. It's okay to daudle with details a bit. You can describe things more and slow down the action so readers have time to acclimate to the story. :]
Okay! Well, I hope this wasn't too harsh! I really like to try to help people improve, but I hope I didn't get TOO nitpicky...I'm so sorry if I did! Don't be discouraged, okay? We all improve as we go along. You're doing quite well already! I'll read the next chapter soon and give you feedback on it too. :] Keep up the good work and you'll see yourself improving in no time!