Emmam, school christmas plays are a joy to see. Because I know my son is all too fond of distraction, I never wave or smile once the entertainment begins. But, when he first comes on stage, I catch his eye, so he knows where I am. Then I keep a straight face (teary eyed) and clap very VERY loudly in all the right places. I try to think of his poor teachers, each trying hard to keep 30 or so children on course. How do they do it?
Also, it's a good idea to make it very clear to your child in advance what happens after. Are you taking them home or not?
Once they see you at the school they can get very upset if you leave again - my son once howled his way through a post-christmas concert assembly when all the parents had left, despite knowing it was only morning and he had a school day ahead of him. (I know this because I hovered in the corridor and tried to comfort him before he went into class).
Hope you have a brilliant time!