Hi alb,
If I was in your shoes … I would probably go with 100% natural Dunlop or Talalay latex in a medium range of firmness. A good cover is important with latex both for waterproofing and to protect the latex from breaking down from exposure to ultraviolet light and ozone.
The other choice I would consider is wool but latex would be simpler to cut and cover to the right size and shape.
I would tend to avoid memory foam or polyfoam for an infant.
Post #4 here has some good sources for latex and I would ask them if they can ship you a piece that is an odd size or a scrap that you could cut rather than a twin size which is usually the smallest they offer.
Phoenix