So you've found your way into the mirror room in Little Nightmares. You've tried climbing just about everything around you with no luck. It turns out that the solution is more simple than it may appear.
When the big monster leaves the room, you will notice that a can falls on the ground. You need to pick up this can and then toss it at the mirror. If you aim it correctly, the mirror will shatter revealing a hidden area behind it.
And that's it. Good luck with the rest of the game!