what kind of mattress did you buy them?

We're buying DD a twin bed for her birthday. Her crib converts to a toddler bed and then a full bed, but there's no space in the bedroom for a full size bed and DS's crib, so a twin bed it is. I found the frame and I want to order the mattress today, if possible (her birthday is in 2 weeks) but I have no idea what is right for a 3 year old... is firm too firm? Will a pillow top offer enough support? I have no idea. TIA!