It is really hard to completely avoid fast food these days, especially for youngsters. When teenagers meet up, they don't want to be eating samosas or parathas or idlis. What they want is to hang out at McDonald's, sipping some chilled chilled colas, nibbling at fries and eating burgers. So instead of just telling your kids to never eat at these fast food joints, perhaps it is better to show them how to make healthier choices (less unhealthy, if you wish) when they eat out.
At a Pizza joint, the best way to eat healthy is to order a thin crust whole wheat pizza with various vegetable toppings. Combos are best avoided unless there are many people to eat all those pizzas! Colas, desserts and sides are best avoided.
When eating at a burger joint, check if they have whole wheat buns available. No matter which burger you order, tell them to skip the Mayo or whatever cheesy sauce they use. Ask them to give the cheese slice a miss too. Ask for extra lettuce and tomato.
Subway fares better when it comes to offering healthier choices. When ordering your sub, make sure you get a protein, lots of vegetables and carbs from mixed grains. Avoid cheesy sauces and sides.
The basic funda is that when eating out with friends, one can still try to make choices that are not as unhealthy. This is what you need to show your kids and encourage them to practice.