Hi coolmmonk,
By “cover” I meant the cover (called the ticking in the industry) of the mattress … not a “cover” you add after you’ve purchased a mattress such as a mattress protector. The cover could be just a fairly thick fabric or have various types of natural or synthetic fibers or some kind of foam quilted to it which surrounds the materials inside and helps the mattress keep its shape.
The cover of a mattress is a significant part of its cost and performance both in terms of how it “feels” and as part of the cooling and ventilation system of the mattress. The materials that are closest to the body will also have the biggest effect in terms of any offgassing and leaching of materials which can be an important consideration especially for a young child that still has a developing immune system.
In addition to the mattress though … it’s also important to make sure that you use a mattress protector which will also have an effect on the feel and performance of the mattress and on it’s cooling and ventilation as well (see post #89 here for more about the different types of mattress protectors). Without a mattress protector to protect against accidents … especially with children … even the smallest stain can void the warranty and in terms of hygiene it’s much easier to wash a mattress protector than to remove a cover and wash it (assuming it can even be removed or washed at all).
A mattress protector is important for both adults and children to protect the mattress against accidents and the natural moisture and body oils that are released during sleep but for children a waterproof cover would be more important than for an adult because of the greater likelihood of “accidents” during the night.
Phoenix