Teaching your kids to be smart shoppers starts with being a smart shopper. You can't teach your kids to be something you're not.
There are three measures I use to define smart shopping:
Smart shoppers don't buy what they won't use.
Smart shoppers don't buy what they can't afford.
Smart shoppers don't pay full price.
Here are some tips to teach your kids how to be smart shoppers:
Let kids have their own money
It is important for kids to learn to manage and spend their own money. This is only effective, however, if it is hard for them to get money. You can - and should - give your kids a small allowance (which is most effective if tied to doing chores), and you should also encourage them to find opportunities to earn money from other people. Never pay your kids more than the neighbors would pay for the same task.
Let kids spend their own money
Especially for things you may not value as much. Encourage your kids to buy things with their own money. It is a good life experience for kids to save up their money for a Kindle or an iPad and to have to make a decision between a $200 Kindle Fire and a $330 iPad Mini. Your kids will never learn to be a smart shopper if they never know what it is to want something without having money available to buy it, or by being forced to choose between two valid options.
Plan with your kids
Before you go shopping with your kids, sit down with them and plan your shopping. If you are going to buy school clothes with them, identify your budget and talk about your choices of stores. Show them the sales you've found to shop and how you found them. If you just drag them from store to store, whether you've planned well or not, the experience will lack any educational experience.
Use the internet together
If you work with your kids to find the best price for stuff you want online, you may find that your kids can help you save money. Born in a digital world, children often have an innate "tech" intuition. Working together, you can teach them good judgment while they help you find the best ways to save online.
After planning, preparing and shopping online, take your kids with you to shop. You don't have to take them every time, but once in a while - especially when shopping for them - take them along so they can see how you work. Be sure to explain your strategies for shopping. Be on your best behavior. Shop with a plan. Stick to the plan. When you feel like splurging, fight the urge with giant pretzels for everyone.
As parents, we all want to teach our children to grow up into full-fledged and high-functioning adults. Life requires good judgment when shopping. If you take the time to help your kids learn to be good shoppers, they'll be much more successful in life.