Full size mattress for toddler

We’re moving our son into a full size bed and looking for a good mattress set. What should we be looking for for a 3 year old? Firm? Plush?

Hi spearlberg,

Post #2 here includes some guidelines for children’s mattresses and also includes links to some of the better forum posts and topics about mattresses and children including information about some of the better options I’m aware of as well.