I agree with both Boo and Stuck but I also ask. Is the child heavy enough to keep the seat down?
I ask as I remember taking my daughter to the cinema to see a Disney film when she was around 4 and she wasn't heavy enough to stop the seat from tipping back. I can't really remember if the seating in a theatre tips up or not to allow people to pass (I got around it by putting my bag full of kiddie "junk" on the seat beside her as "ballast")
My grandson was 3 and a half at his first pantomime, and if he had been any younger things could have been difficult. Even then, he had very little idea what was going on, or how to sit still and try to follow the action, let alone the story. I'd give the child you mention another year unless s/he is both abnormally intelligent and well-behaved.