Designed for students too small for the regular clarinet the Kinder Clarinet is smaller, lighter and has a simplified key system. Just as violins are built in smaller sizes for young beginners, the kinder clarinet is effectively a 3/4 size instrument. Its sounds higher - it's pitched in Eb rather than Bb. It also has smaller sized finger holes which encourage a correct and natural finger position, rather than the stretching and straining often seen when children try to cover the lower holes on a Bb clarinet.
Starting lessons on the kinder clarinet allows a child to learn clarinet technique but on a smaller instrument. The embouchure (mouth formation) is the same and they can learn to produce a tone using the reed. Although the key system is simpler, the majority of fingerings are identical to the Bb clarinet: Its really only the keys needed for more advanced playing that have been removed as this helps lighten the weight of the instrument. Once correct finger placement, balance and embouchure have been developed the transition to a regular clarinet is much simpler.
For parents not wishing to buy a Kinder clarinet I have one instrument that is available for hire.