Hi NYCmom,
Post #2 here includes links to most of the better threads around the forum that discuss mattresses and children and they include links to many good options for children as well.
For local options … post #2 here includes the better options and possibilities I’m aware of in and around NYC and post #7 here includes a more categorized list that has more detailed descriptions for some of them.
Children generally need a firmer mattress than adults because their growing bodies don’t need as much “cush” and need good support. They also don’t need as much thickness because of their lighter weights.
Depending on how you feel about different mattress materials … innersprings, natural fibers, and latex are all good options for children’s mattresses. If you are comfortable with polyfoam in a mattress then I would make sure it was CertiPur certified for harmful substances and VOC’s. I would avoid memory foam for children.
Phoenix