IKEA does have a children’s mattress - the PLUTTEN foam mattress.
This children’s mattress is made of foam, compatible with extendable beds, and recommended for children above 3 years.
Some of the features of this mattress are:
One mattress offers three different lengths (from 130 to 165 or 200 cm)
Smooth surface on both sides
Washable and child-safe zippered cover
The children’s mattress offered by the brand is a good and affordable choice.