As an ex primary teacher I really wouldn't mind a parent bringing this up.
There's currently a thread where a poster has been emailing the school about replacing their child's sticker. Honestly that's the kind of stuff that parents are complaining about on a daily basis. This isn't like that at all.
I think "that parent" and "has enough going on" gets wheeled out too much here. Presuming you don't complain constantly about tiny things and presuming it's something your child is actually upset about I really wouldn't mind an email like,
"Hi Mrs Bytes.
I know you're working really hard on the nativity and getting everything ready. Derek has been feeling quite upset about it this week. He was hoping for a speaking part in reception but didn't get one. He noticed that most year 1s get the speaking parts so was really looking forward to it this year. Now he's not got one he's feeling really disappointed especially as it's the last year they get this chance.
I know it might be too late but if there is a chance he can get just a single line to say then I think it would really help him. I know he's not the most confident but I think this is a chance for him to improve on that as he would be practising projecting his voice and really would want to do well.
Please let me know what you think."
Honestly it happens once a year (twice ever) and it's a big thing. It does sound weirdly unfair to pick year 1s and some reception (why not just stick to all year 1s who want a part then use reception for any left over roles?). Some kids won't speak their lines as well as others but if he's lobster #3 it doesn't matter. As long as you're not expecting him to be Joseph or anything.
As a teacher I did have a lot to do and constant complaining about tiny things or demands from parents was hard work but (again assuming this is coming from him not you) if a quiet, well behaved child was going home upset and feeling bad about themselves at 4 years old because of this I would want to know.
I still remember being a minority of kids not picked for choir at school and i felt awful and it put me off singing thereafter.
Giving a child the message that he's not good enough to perform if that's what he wants to do doesn't sit right with me. And I say this as someone who has added the most convoluted lines and parts to school plays so every child who wanted one had a character to dress as and one like to say.
Unfortunately it's just widely accepted with things like music, drama and art even from a young age that the kids who are already good at it get encouraged and the ones who are not are left behind. In other subjects like Maths it's the opposite and the less able children would get picked to participate in events to build their confident and help them improve.