I think DS was about that age when my sister took him to McD's with her friends and their children - without my prior knowledge or consent! Her friends were doing it as a nice treat, it is a nice treat, their children are same age as my DS... and while I was horrified, I didn't have a go. She was giving DS a nice treat, one that wouldn't harm him (like cow milk does at least!) and as a one-off it isn't a problem. He got to play longer with his friends, and when we visit my mother there are few friends for him, so it's nice he gets included in some things.

DS is so used to fresh veggies and home-made chips and generally nice food that he wasn't fussed. He doesn't ask for fast food (we've done it a couple of times since then, mostly on long journies where there's no-where else to eat) and isn't overly fussed about it. He doesn't even think the packaged fruit is that nice, he's too used to the real deal. I think the meal deals are fries, burger/chicken nuggets, ice cream/fruit and juice/water/carbonated drink.

So I would let your DS go - but take some fresh fruit in your bag just in case!