A vaporiser heats water to produce steam. The warm steam then penetrates the air in the room. When the humidified air is breathed in, it helps loosen congestion in the lungs and bronchial tubes and eases the pain of a sore throat.
A humidifier works differently, only in the fact that it creates a cool mist instead of a warm steam. The cool mist provides the same benefit as a vaporiser, by humidifying the air in the room.
There are also over the counter products available that can be used with your vaporiser or humidifier to add vapours to the air.
It’s really a matter of personal choice weather you use a vaporiser or a humidifier. But in either case, it is vital that the unit be keep extremely clean. If bacteria are present, it can be vaporised and subsequently breathed in by your child, which is liable to cause other problems.