The airlines are being hit extremely hard with the cost of fuel. You could say that they need to develop more efficient planes, but all that takes time, money, and engineering. In the meantime, they need to deal with the state of the economy as it is.
Many people are business travelers, and tend to take essentials only - they need to travel as lightly as possible. On the other hand, families going on vacation do, as Jase has said, bring along everything except the kitchen sink. If the airlines spread the cost by increasing the cost of tickets, it penalizes those traveling lightly. Many business travelers are already paying increased prices for "Business Class" seats. It seems fair to charge for each bag, and to begin limiting the size and weight of carry-ons.
The heavier the plane, the greater the amount of fuel necessary to reach the appointed destination. Just consider it a way of "forcing" people to be environmentally aware of how much they are taking from the earth by their materialistic dependence on their "stuff."
If you really think about what you "need", you may find that you have less hassle upon arrival as well. Trips should be a bit of an adventure anyhow, and your children will learn that sometimes "less is more."