A debit card is basically a way to allow you to access money that is already there in an account. If however much you're trying to spend is over the amount in the account (say a $10 purchase but there's only $8 in the account) it won't let you spend.
Now honestly? I'm a fan of kids getting one of these as soon as they're old enough to get paid for anything. You pay them for house chores? Debit card. They babysit or mow lawns or whatever and get paid? Debit card. Teach them how to do online banking since the internet is such a huge part of life now, show them where their deposits and withdrawals are, teach them how to start keeping track of their finances. That's a life skill I think a lot of us are not taught, and it's good to teach at a young age. It also helps them feel more like an adult and like they are in control of things to an extent.
This can also be a segway into a bunch of other lessons about saving for what they want, keeping track of where their money is going, and general good life skills. As long as a responsible adult is helping them manage it I really don't see a problem.