Might this be Dark Oracle (2004-2006)...?
Dark Oracle is a Canadian-produced TV series that premiered in 2004 on the popular Canadian channel YTV. It was created by Jana Sinyor (former writer for Degrassi: the Next Generation and creator of Being Erica), and co-developed by Heather Conkie. In 2005, Dark Oracle won the International Emmy for Best Children's and youth program.
Omen returns in the last episode, when Cally and Lance lift his curse once more, in order to get rid of Violet and Blaze. He makes an attempt, but fails. He tries to get revenge on Doyle once more at a Masquerade ball, but is confronted by the Stone siblings. They chase him throughout the ball until Omen is alone in a room with nothing but a mirror. He hears the voices of Blaze and Violet and is pulled into the mirror. At the end of the season finale, Violet and Blaze sneer at Omen saying, "Welcome back, Omen".
After a plan to rescue Lance fails, Omen reveals his true feelings to Cally. Doyle, Vern and Omen work on a Power of Three spell to summon and kill the Puppet Master (the opposite of Doyle). Once Lance is freed, the Puppet Master attempts to use an erasure spell on Cally. Omen sacrifices his own life for Cally and disappears. Whether he returns to the comic or dies is unknown. There is a possibility he could be alive as someone says "Cally" in a very raspy voice through the mirror after Cally gives her final goodbye to the mirror. Whether this is Omen's voice or not is unknown.
It definitely has scenes with young characters interacting with people inside of mirrors. Both this show and the first season of Animorphs aired on YTV in Canada.