This is James Potter, Harry and Ginny's oldest son. In the book version of Deathly Hallows, the three children have the following exchange, which more-or-less matches with the dialogue you describe:
"It won't be long, and you'll be going too," Harry told her.
"Two years," sniffed Lily. "I want to go now!"
The commuters stared curiously at the owls as the family wove its way toward the barrier between platforms nine and ten, Albus's voice drifted back to Harry over the surrounding clamor; his sons had resumed the argument they had started in the car.
"I won't! I won't be a Slytherin!"
"James, give it a rest!" said Ginny.
"I only said he might be," said James, grinning at his younger brother. "There's nothing wrong with that. He might be in Slytherin"
Harry Potter and the Deathly Hallows Epilogue "19 Years Later"
Although I haven't seen the movie version, the picture of James Potter on the Harry Potter Wikia seems to match your description:
The first child you describe is Albus Severus Potter1, Harry and Ginny's middle child, who is starting his first year at Hogwarts:
The second child you describe is Lily Potter, Harry and Ginny's youngest:
And here's a picture of the kids waving from the Hogwarts Express2:
1 That poor, poor child
2 As to the identities of the girls in that picture, the one nearest the window is Rose Weasley, Hermione and Ron's oldest child. The other two are unidentified as far as I can tell, but one of them is possibly Bill and Fleur's daughter Victoire Weasley. However, no actress was credited for the role, so we can't be sure. I've posted a follow-up question, so please weigh in if you have any more info