For example, it is not OK to let your 5 year old stand on the seat/bench. That's the same as an adult standing up for the duration. Being a child isn't the same as being see-through.
It is not acceptable to risk bringing a loud toddler and then when said toddler decides that running around and screeching is what they want to do most, stay put. Bring them out. I don't care if that means you miss the show - the alternative is that a hundred + other people have the show disturbed. Do the right thing.
It is acceptable to do a lone risky descant during the "Sing choirs of angels" bit of "O come all ye faithful". No-one went for it this year, and I was gutted.
What else?