Many children do not understand about $$$. Set down with them, show them how much you make, how much goes to taxes, food, ulities, house, etc.
When they ask for something you can get your chart out and find out if there is $$ left for there desires (which often will be NO)
Seeing it on paper will stop the crying "Why not"?
This also will teach them how to budget at what ever age they start to receive $$.