Hi Toddandleah,
As long as the sleeping surface is relatively firm and has some flexibility to it then it can be suitable and the choice of materials would be more about personal preference and durability than it would about one being “better” than another. The spines of children are still growing and their spines are more flexible and subject to alignment issues so good alignment is always important with children’s mattresses. The relative “safety” of materials or the choice between natural or synthetic materials also plays a role in many parent’s decisions but the answer to the question of “how safe is safe enough for me?” is a complex question that only each parent can answer on an individual level. (Post #2 here has more links to many good sources of information about this) even though there may be no definitive answers.
Natural fibers (and semi synthetic fibers such as viscose types of materials) are great at maintaining the sleeping microclimate in terms of humidity control and temperature regulation because they are very good at wicking moisture and ventilation but they are also more costly so there is always a tradeoff between natural materials or better performing and more durable materials and cost.
The children’s threads I linked earlier go into this more and include some good sources for children’s mattresses as well but for those who are more concerned with more natural or “safer” materials then innersprings, latex, and natural fibers can all make good choices yes. If you do choose to use polyfoam because of the lower cost (and of course in some cases this is necessary) then I would make sure it is CertiPur certified for harmful substances and VOC’s. Natural fibers are also very durable and in most cases are more durable than synthetic fibers (although all fibers will compress and compact down over time).
Phoenix