They mostly are broken bones, such as arms and wrists, but some kids have broken their necks and been paralyzed.
Even adults are getting hurt jumping on backyard trampolines.
On The Early Show's ConsumerWatch report, correspondent Susan Koeppen has advice on right and wrong ways to bounce.
The big no-nos are somersaults and flips. They can cause serious injury.
Five-year-old Sara Swartz has good reason not to like trampolines. Last summer, she was jumping on one in the backyard, when she landed the wrong way. It is easily visible in an X-ray where her arm snapped in two.
Her mother JoAnne Swartz says, "My husband came in with her in his arms and she was screaming, and I could see that her upper arm was deformed, and I knew it was broken."
After she was rushed to the hospital, Sara spent six weeks in a cast, and two months in physical therapy.
JoAnne Swartz notes, "She didn't hit anyone. She didn't fall off the trampoline; there was nothing out of the ordinary; there was no collision."
The Swartz's trampoline came with protective netting and padding. Joanne Swartz says it looked completely safe.
She notes, "This accident happened when the children were well supervised; everyone was watching them. They weren't doing anything extreme, and yet, she had a very, very serious injury."
In 2003, 87,000 children were sent to emergency rooms with trampoline-related injuries; most occurred at home in the backyard.
At Children's Hospital of Philadelphia, doctors see three to six trampoline injuries a month.
Pediatric orthopedic surgeon Dr. Denis Drummond says, "It's a catapult, and it projects you into the air, and you got to fall just as quickly as you go up."
He treats plenty of broken bones from trampolines. He says some injuries can even lead to paralysis.
"You can always heal a forearm fracture," Dr. Drummond says, "but you can't heal a spinal cord."
Mike May from the Sporting Goods Manufacturers Association says, "Trampolines are not inherently unsafe for kids."
When used properly, trampolines are a safe, outdoor activity, but parents should know, they're not toys.
May explains, "When you use a trampoline, the importance of maintaining a focus and concentrating on the activity is no different than driving a car. If you're driving a car and are distracted, you might have an accident."
To reduce the chance of injury, May says there should be no more than one person at a time on a trampoline. Children younger than 6 should never be allowed to use them, and an adult should closely supervise any child on a trampoline.
JoAnne Swartz says adult supervision didn't prevent her daughter from getting hurt. She took her trampoline down as fast as it went up, and says she'll never let her kids near one again.
"Anything can happen," she says. "And do you really want to take the chance that your child could be seriously injured like that?"
To other children who are bouncing on trampolines, Sara says, "Don't jump on trampolines 'cause you might break your arm like me."
Even the insurance industry is taking notice of trampolines. Carriers call them an "attractive nuisance," just like backyard swimming pools.
They recommend putting up a fence around your yard if you have a trampoline. And make sure your trampoline has a safety net.
If you have a ladder to get the kids up, take the ladder away. And when it is not in use, lock the net up.