I live in a town that doesn't have any chain fast food places. It has restaurants (pubs), but they tend to be a little on the expensive side so we don't go very often. There is a take-away place, but the food there isn't particularly appealing. We go to the city about once a month, or once every three weeks or so now and we will usually drop by a fast food place then, but that's not too bad.
If you go, look for the healthy options ... grilled chicken instead of breaded chicken, salads, etc.
You said ... "Sure - if you have a small order of fries at McD's you won't get sick. But come on - who really goes with the family and just eats a small fry?"
Well ... I do. But as for you ... you're the father, right? Shouldn't you be setting an example for your kids? You don't want them to be those obese kids in all the statistics do you? Choose the smaller portions, choose the healthier options ... and if you (or they) are still hungry later in the evening, eat some fruit.