Most often airlines do not have many food restrictions for on board but do be considerate about taking peanut and nut products as if there is someone with an allergy near you, you will be asked to keep it in your bag.
If you are traveling within Canada, you do not have to worry about what you take (unless it is a liquid, be sure to adhere to security regulations, exceptions are made for pumped milk and liquids for children under 2 years old) so you can take sandwiches, cheese, crackers etc. My son thinks eating during flight is a special kind of picnic. He gets to use the tray-table for his food and little toys! I like to take their favourites and apples and grapes are easy to travel with and don't require refrigeration and aren't messy.
If you are traveling to the USA and pre-clearning in Canada you will have to declare any food items you have with you and some items are not permitted. (Consult the US Border and Protection website). If you are traveling internationally, it is ok to take the food on board with you but it should be eaten or thrown out before you land and clear customs.
We also purchase food during flight, but it's best to have a snack for times when the kids are hungry (and it's less expensive that way)
Bonne Appetit. Happy Traveling!