Any student at an Ivy League University - usually with insanely high test scores, gobs of leadership and sports positions, loads of extracurricular activities (like forming businesses and doing research on cancer cures, which is where the "miracle" part comes in) you couldn't think of, etc.
Sometimes very obnoxious, too. They seem to be living miracles to the common folk as Ivy League Universities overhype their "reputation" so to try and attract the overachievers before deliberately inflating the grades there so to keep looking good as paper tigers.
Can be used as a derogatory statement but is usually a compliment
My cousin Traci is a miracle kid. She was the captain of her lacrosse team and our school president, and was involved in practically every honor society in school. She nabbed every academic award you could think of and graduated as the sole valedictorian of our school. On the side, she had her own small web-designing company and worked with a family friend on finding a cure for ovarian cancer. She also got 800s on all her SAT IIs and a 2350 on her SAT, and she wowed all the alumni interviewers with her impressive verbal skills honed from her manipulative school campaigns not only to become a leader but for various movements like recycling and more lavish pep rallies and a relaxed dress code. And thus, she was accepted into Princeton University. It's no wonder I or the family never got to spend time with her.