I had a friend at Uni who had grown up in a strictly vegan household, and as soon as she got 'out into the real world' on her own she would GORGE herself silly on every kind of junk food she could find, mainly because she had never tried any of it before. She ended up with terrible skin and quite overweight because of it.
So my personal opinion is that you should expose your kids to all types of food, and yes - takeaways should be treats rather than the norm, but hey we are all human and have days where we just cant face cooking! My nephew is 2 and we sometimes have fish n chips at our house when he is over, he has a few chips and the fish minus the batter. He thinks its a great treat!