You should NOT use a mattress for a second or third child if :-
1) You do not know its history how it was looked after and how it has been maintained and cleaned
2) If the Mattress does not have washable covers and the core is not protected.
3) If the mattress has the slightest dip or dent, - not supportive.
4) If the mattress is yours and you stored it in the loft and did you seal it in a plastic bag and it is not thoroughly washable, and does not have protection to the core.
You can use a mattress for another child if :-
1) You know its history and how well it was looked after
2) The mattress has a protector (preferably wipe clean and or washable) that will prevent body fluids getting to the core (foam or springs) and the protector sits under the cover and over the foam.
3) The covers have been washed regularly at 60°c the temperature requite to kill dust mites.
4 ) The mattress has no visible dents or dips has been well maintained and turned frequently
5) The mattress has been stored clean and sealed in plastic. All covers or toppers protective core cover machine washable at 60°c . A fixed waterproof protective base cover can be cleaned with an anti-bacterial cleaner like Milton.
NB.As foam ages it Oxidises, goes yellow, this is perfectly normal and does not affect the performance of the foam in any way whatsoever,
Cot mattresses in the UK use a quality of foam called 24 / 130 sold by the major dept stores and trusted web companies this foam that covers springs will last about 1- 3 years depending on how it is maintained and rotated and the heat, weight, activity and dampness of your baby.
Nighytnite® use foam with a strength 30 /150 and 50 / 215, this is a massive step up in quality. Nightynite foam encapsulating pocket springs will give Excellent comfort, support and durability.
The Excellence mattresses are guaranteed 7 years they have full waterproof protection and machine washable covers. You can certainly use these mattresses for a second child if follow the instructions that come with the mattress.