We are going to start with this Speedy 1 made in Germany. Very convenient and here is why: You can pedal or rather move the pedals with your hands, it is a unique contraption that puts together a tricycle and a wheelchair, but you can also separate them as you see here. You can find more information at Speedy Bikes. I forgot to mention that you can also find them with pedals for your feet...(sounds kind of weird). In any case, there are all kinds of combinations.
Ok now we'll talk about another cool device that I find very much fun and useful for special needs persons, but also for children that are trying to learn to ride a bike. It is the Gyrowheel. The company is actually called the same. The way it works is by introducing a gyroscope inside the wheel. The rotation of the gyroscope or a wheel inside a wheel produces upward balance on the wheel, therefore it does so on the device attached to the wheel, which in this case would be the bicycle. It does use batteries and it comes in 12" and 16" diameters. I frankly don't think you should have to spend the money for one of these to train a child without developmental or mobility problems, but if you think about it, a person with a disadvantage would perfectly benefit from it.
The BuddyBike on the other hand, uses no batteries. You have to ride it. Quite convenient because (according to me) of these reasons: You only have to get one bike, your child is close by and in within your reach. Your child gets to feel the direction of the handlebar and probably also feels in control.
I do not work with people with special needs and if for any reason I am being politically incorrect, those of you that have more knowledge in this field should feel free to correct me.