Eating out is generally inexpensive in France, outside the large cities. A decent two course lunch can usually be had for about 12 euros. If that is still too dear you can track down a restaurant in a supermarket e.g. Casino and eat well for even less. If it is too little you can pay more and eat better. Ask for recommendations.
Bread is usual, butter on bread is not.
Don't order something unless you know broadly what you are going to get - either ask, or avoid it. If you don't know that steak tartare is raw beef mince you may be disappointed when it arrives, for example. It will still be good food but may not be what you choose to eat or are familiar with, and may spoil your experience.
Tipping is officially unnecessary, being now included in the bill, but most people still do, especially for good service.
French children in restaurants are usually well-behaved, not running around creating havoc. Other countries are sometimes different.
Good food and good wine are generally appreciated and widely known about.
Fast food restaurants also exist, though not often in large numbers.
Eating at home
We eat creme fraiche with fruit at our house. A French child was appalled and asked why we were putting 'pasta sauce' on our strawberries.
French are always amazed to see us put butter on bread at home
The French children eat an enormous amount of chocolate spread (nutella), and often eat a lump of baguette with a bar of chocolate in as their 4pm snack - 'gouter' as it is called, and apparently impossible to do without.
Note: there is a related blog post that covers a great deal of French eating habits, with information and several useful suggestions for entertaining: see french eating habits.