A toddler mattress needs to Durable offer Excellent Support and Hygiene It should last for at least 5 years and see a baby through newborn infancy and to a toddler
A totally foam mattress will not give the support and wear compared to a mattress with springs
Memory foam is definitely not a good idea not for child under 3 it is too hot, there is a danger of pooling, and it does not give good support .A child body is changing all the time and the surface need to be firm and flat.
A toddler mattress should be able to take a lot of punishment children will jump up and down on it, dive on it, and it should not dent.
A toddler mattress must be able to be easily cleaned and still offer comfort and temperature control. During the potty training periods there will be the odd accident but it is still possible to have you child sleeping on a breathable welcoming luxury cover rather than a plastic waterproof sheet.
The density of the foam covering the pocket springs is key to the support comfort and durability
Every sprung or pocket sprung mattress has the springs encapsulated in the foam mattress The springs are 6 cm deep the mattress should be 10 cm deep to conform to British standards . So that means 2 cm of foam either side of the springs.
this foam take all the punishment weight of the child from above and the springs below.
look for a mattress when the foam used is a high density foam this will give good support will last 5 years and offer a comfortable sleep.
The minimum you should accept is 33 weight foam there is a mattress made with 50 weight foam. ask for the specifications before you buy
Here is some information to explain how foam is categorised