Child headphones are, like I said, smaller to accommodate the smaller cranium sizes of youths making it much more comfortable for a child to wear than larger adult sets. This isn't a big deal for short-term use, but for longer periods like watching a movie in the back seat of a car on a long car trip, it can make a difference, though the exactly amount or type has not been studied thoroughly.
They also have built in volume controls. This is by far my favorite feature of child headphones. I can't tell you how many times I've accidentally blasted my own ears starting some music when the volume had been turned up without me knowing. Built in volume controls mean that even if the device they're plugged into is set to play loudly, the headphones have a maximum level that will lower the volume to a safe level for their ears.
Lastly, they look cool! I mean, check out this pair by DreamGear. With cute pictures and characters on them, children often get attached to their headphones as it allows them to have their set that. This means they'll (theoretically) stay away from your expensive Bose set, and opt for their own.
Practical Tip of the Day:
Be careful giving your child any electronic device or accessory and leaving them unattended. Smaller children may chew on cables or get tangled in the wire. Always exercise common sense and caution when giving your child a new item. I personally did not let my child use her headphones unattended until she was five, and I still do not let her use them for more than 10 minutes at a time except for special occasions like the aforementioned long car trips.
