I was reading out your posts and laughing about how ridiculous you were being when my dh pointed out a similar scenario in my past.
My dd was looked after by my MIL to the age of 2 for one day a week. I never liked the arrangement much, and she did lots of stuff that really ticked me off.
The incident in quesiton, however, wound me up beyond reason, much as your McDonalds situation has.
My Mil used to dress my child in different clothes...photograph her, then put her back in her own clothes for when I picked her up. I woudn't have known about this but she accidentally gave me the wrong pack of photos one day when I was visiting and there was dd in clothes I'd never seen before. It sounds really petty now, but god, it drove me insane.
It's a control thing. You have the things you want for your child and you make this clear to your relatives. They then disregard these wishes and do stuff that you find distasteful or ridiculous.
I'd say there may be other things going off here, outside the McDonalds incident.
AYBU about the McDonalds? To some degree yes...it's just a one off. Are you being unreasonable about the fact that your wishes have been flouted? No. You're just having that perfectly illogical, irrational reaction to other people looking after your child in a way that seems inappropriate.
You'll look back at it when DS is 6 or 7 and realise that it didn't really matter much in the scheme of things. But you're entitled to your annoyance right now.
Thanks to DH for taking my eye off the McNugget debate (and it sounds like your DS does like meat, so I woudn't rule that out of his diet unless you really have to).