If you ask nunchaku practitioners if they've hurt themself during practice, the answer is overwhelmingly yes. While nunchaku are easily one of the most visually impressive weapons when used properly, they have a high learning curve and are famously hard to control. The result is a lot of inadvertent bumps and bruises.
To minimize the damage of those early-learning errors, some individuals choose to use foam nunchaku. Regular nunchaku are made out of dense wood or hard plastic and are unforgiving when hitting the human body. Foam is a little more easy going.
When spinning nunchaku, some of the most common areas where people strike themselves are the knees, hands, and head. The knees and head come during wide swings and the hands when trying to catch. In fact, many head bumps come during missed catch attempts.
Foam not only protects your body but is also light weight and easy to manipulate. Getting used to the heft of a realistic pair of nunchaku can be daunting, especially if you have pre-existing shoulder or wrist injuries. But swinging "foamies" is a breeze, which makes them especially appealing to children with generally weaker physiques and less developed motor skills.
There is one important thing to remember though - at some point it is highly recommended that you stop using foam nunchaku. People who use them for too long gain a false sense of confidence and bad feel for how real materials like wood behave when swung and manipulated. If you use foam too long you'll find transitioning to wood a difficult and once-again dangerous proposition.
In my mind, it's completely acceptable to use padded weapons when beginning your training, but once that initial fear is gone and you've gained a certain amount of control it is wise to switch to the real thing. You may have to take some minor bumps and bruises along the way, but you'll avoid the real bangers that make for hospital or dentist trips.