Best mattress against mildew

Hi. I smell mildew in my children's room despite of good ventilation. I'm thinking to buy a spring mattress as I'm trying to avoid any foam. But I'm not sure if that's a good idea, as some kind of foams are anti-allergy etc. Also the depth of it shouldn't be more than 15cm as it is going to be on a bunk bed. I am very confused which mattress to get for my children. I hope to hear soon from you.
Thanks