You haven't missed much. Invariably your grandchild will somehow be hidden behind the biggest boy in the school. It will be unlikely to be anything to do with the Christmas story, but will feature children dressed as presents, elves, Christmas trees and villagers. Mary and Joseph may make a cameo appearances towards the end.
The music will be played via some electronics device, with electronic children singing on them and will feature songs you've never heard before -and provably wouldn't wish to hear again which sound as if they've just been make up by some company that specialises in trotting out these sad little productions year after year, so that teachers can cram the whole ' Infant' school aka Key Stage 1 into the one production, hence the 23 donkeys, 16 Christmas trees and 132 villagers.
In all likelihood you will have provided the costume and will anxiously be checking that other grannies haven't outdone your efforts to make little Sammy look like a passable Christmas pudding.
Of course you won't be able to hear your GC utter their one liner as somebody' s baby will start to wail just at that point, despite the HT having asked that no babies or kamakasi toddiers be in the audience.
At the end the Assistant Head will stand up and congratulate all the little moppets for their wonderful performance and the teachers for all their hard work in putting such a magnificent production together 
You will find it difficult to get up to leave having been forced to sit on a tiny wooden chair for the best part of an hour and will not confess to your GC that you couldn't see him/her because some 6' 4" father was using his mobile phone directly in your sight line to video the 8th Christmas tree from the left.
You will report back to your grandchild's parents how amazing little Eddie was and you will probably, by then, have convinced yourself it was true,,