Hi ScooterBrown,
Post #2 here and the topics it links to has much more information about mattresses and children and choosing materials that are “safe enough” for children. It also includes a number of links to the better forum posts and topics about mattress and children as well including several links to topics about crib mattresses that can help you decide on a crib mattress that is the best “match” for all the criteria that are most important to you.
While these are general guidelines and not “exact” recommendations because there is no single firmness level that is always best for all children and different people or different manufacturers can have different definitions of firmness … babies and infants will generally need a firm to extra firm sleeping surface while toddlers will generally need a medium firm to firm sleeping surface and older children that have transitioned to a larger mattress and are out of the crib will generally do best on a medium to medium firm sleeping surface.
Any combination of materials that are in a suitable firmness range, are durable enough (especially if you plan to use the same mattress for other children), and that are “safe enough” for your criteria, and that are in a budget range that you are comfortable with would make a suitable choice.
Innerspring mattress (vs mattresses that use various types of foam or coir as a support core) will tend to have more “bounce” which can encourage using the mattress for a trampoline (which can be somewhat risky) although they are certainly a suitable choice in terms of suitability and durability.
Many innersprings also have an edge support system which may be important if you If you tend to sit on the edge of the bed with your child for “story time” so it may be worthwhile to check for edge support although it won’t make a difference in terms of sleeping.
Latex will be more resilient or springy than polyfoam or coir but not as springy as an innerspring.
Innersprings would be the most breathable followed by coir followed by latex followed by polyfoam but the cover and quilting materials will be the most important part of the humidity and temperature regulation of the mattress so once again any of these mattresses would make a suitable choice.
Latex will tend to be heavier than innersprings which in turn will tend to be heavier than coir or polyfoam if weight is an important consideration for you.
All of the mattresses you listed use materials that for most people would be “safe enough” and don’t contain any harmful substances or VOC’s and are in a suitable firmness range so they would all make a suitable choice for a crib mattress although the Royal-Pedic is in a higher budget range than I would consider compared to other similar mattresses because IMO there is no meaningful reason to purchase a crib mattress in this budget range either because of safety or durability benefits.
The Greenbuds mattress has a very firm and a firm side so you can use the firmest side when they are a baby or an infant and then use the firm side when they become a toddler but all of them have a firm sleeping surface that would be suitable for babies, infants, or toddlers until they move out of their crib (usually between 18 months and 3 years old).
The choice between cotton and wool would be a preference choice since both of them would be breathable and suitable for a child (as long as you are also using a waterproof mattress protector to protect the mattress against accidents) but wool does a better job of regulating humidity, can hold more moisture and humidity without feeling damp, is more resilient and generally “packs down” less, and is a better temperature regulator than cotton … but it’s also more costly.
Phoenix