Everything in moderation. A once-a-month trip to McDonalds is not going to kill them, and it will help teach them about moderation.
The problem with not allowing something at all is that, when they are young adults, they may rebel against the abolitionist environment they grew up in and eat there 3 times a day.
When our kids were 10-18, we went out to dinner every other week – twice a month. We rotated who got to choose, so everyone got a pick every 8 weeks. Obviously my wife and I chose healthier places than they did, but you may be surprised that most of the time they didn’t choose fast food either. They’re both in their 20s now and healthy as horses.