Hi wave,
I personally would tend to avoid memory foam with a younger child whose immune system is still developing. This is because even though it may be regarded as “safe” for a child (if it meets the CertiPur or Oeko-Tex or other standards) … I still don’t believe it is a good idea to take the chance of exposing a child to the ingredients in all memory foams until they are much older (see post #2 here).
In addition to this … children spend more time on a mattress, have a much more flexible spine that is less curvy than an adult, and they need to be better supported than an adult by the surface layers. They also do better with more freedom of movement than even more responsive memory foam can provide. Finally … the microclimate of a memory foam mattress is not as breathable and temperature controlling as other more breathable materials and I believe that children need every advantage we can give them while they are sleeping, growing, and developing. This is in addition to any fire retardency materials that may be added to memory foam or a mattress.
So while I understand the desire to give a child a more soft and cushy mattress … it is not a good idea IMO to use memory foam until they are much older. I would focus on more natural materials that provide more of what she really needs, even over what she may prefer, and which has much less risk over the longer term. Kids often prefer things that are not their best choices … and I really would focus on healthier materials and construction.
In a full size mattress … I would suggest that if you possibly can that you consider a slightly higher budget that would make it possible to go with either a more natural material like a single 6" core of Dunlop latex with a wool quilted ticking or an innerspring with natural fibers or latex. If this is not possible … then at least a higher quality polyfoam mattress or polyfoam/innerspring mattress would be more suitable than memory foam (although the polyfoam still isn’t as healthy as more natural alternatives and comes with the risk of additives that wouldn’t be in more natural materials).
If you are OK with a twin size … then something like this may be ideal.
Products - IKEA is a little over your budget but would also make a good full size choice. Something like this may work as well but is also over your budget in full size.
Mattresses that are similar to these may be available from some of the manufacturers on the Atlanta list and even for a child’s bed they would be good sources to talk with over the phone.
Some polyfoam/latex hybrid choices that would be worth considering include …
https://www.amazon.com/gp/browse.html?ie=UTF8&marketplaceID=ATVPDKIKX0DER&me=APCUWEOU23BHI
A futon that used healthy materials would also be a good choice and post #2 here includes some good options that use the same materials as a “non futon” mattress except in thinner layers and may be worth considering.
Google searches on terms like “healthy child’s mattress” will bring up a lot of choices but it will take some research to find something that is both healthy and inside your budget range in a full size as higher quality materials like wool, cotton, and latex can be more expensive.
But the first place I would look is the local manufacturers on the list.
Hope this helps … and as I come across more options in your price range I will add them here.
Phoenix