NEW YORK (CNN/Money) - The lazy days of summer are nearly at an end. Time to trade in trips to the beach and barbecues for science fairs and algebra.

It's time to hit the stores again, too.

For the start of a new school year, kids usually need (or want) it all; from tennis shoes and backpacks to the latest rage in teenage fashion.

The National Retail Federation reports the average household with school-aged kids will spend $442 on back-to-school expenses this year, down $17 from last year. But kids chip in, too.

The NRF notes that nearly half of parents with children ages 13 to 17 say their kids plan to spend some of their own money on back-to-school goods. That makes end of summer trips to the mall a golden opportunity to give your kids an important lesson in money management.

"If a parent can ballpark the cost of back-to-school expenses, they can tell their child, 'Hey, I have only so much for all the items on our list,'" said Debbie Pierce, a spokesperson for the Young Americans Bank, which caters to the under-21 crowd. "That way, parents and kids can choose what to buy together."

Keeping spending under control starts before you hit the stores. Together, you and your child should make a list of what he absolutely needs and what he simply wants. Be sure to check in with your child's school on dress code policies before purchasing clothing. Some schools require a uniform. Others send out a list of required items.

According to Money Management International, some of the items that often get overlooked include a yearbook, field trips, dry cleaning for extra-curricular uniforms, school dances/events and immunizations

"In the store, kids can put what they need and want in the shopping cart," Pierce said. "If the prices add up to more than the budget, children can help choose what should be put back."

Next, take a look at how much you can afford. Set a realistic limit on clothes, school supplies, or other expenses like sports equipment for each child. Let your kids know how much you'll spend, so they're aware of the budget well in advance.

"Sit down before you go shopping and tell kids what you have available for school supplies," said Dara Duguay, executive director of the Jump$tart Coalition for Personal Financial Literacy. "You don't want to get into a discussion about spending when you're actually in the store."

Most importantly, stick to your guns. If your daughter has her heart set on $80 designer jeans, she might reconsider if she knows you aren't spending more than $100 on her jeans and T-shirts. Or she may be willing to contribute the difference from allowance or summer job savings.

"If the kids want something expensive and trendy that blows the budget apart, don't say no outright," Duguay said. "Instead, ask if they're willing to contribute. If kids give some of their own money, it's a good compromise."

And while you're shopping, be sure to keep things in perspective. Sometimes, the problem isn't the kids. If you feel a little too charmed at the thought of outfitting your child with the best goodies, the budget might help keep your spending in line, too.

If that's the case, try to stay off the plastic. Back-to-school sales offer deep discounts on fall and winter clothes and even sales tax holidays in some states, luring consumers to buy more than they had planned. But you'll end up canceling out those "deals" in interest payments if you charge more than you can pay in full the next month.