Sydney was born in London on 30 November 1991. She first attended Broomfield House School in Kew. She got her parents to take her to the children's agency Young'uns. After her first audition she was offered the role of Cosette in "Les Misérables". The next year she had four performances per week at London's Palace Theatre while still doing schoolwork. Sydney then auditioned for the Sylvia Young Theatre School. She was accepted and started in September 2002.