Hi DGow,
Outside of the tutorial post … the first step would be to decide on “how safe is safe enough for me?” which will have a direct effect on the materials you are comfortable having in your mattress. Your budget is probably too low for any type of “organic” mattress if that’s an important criteria for you but most people would be comfortable with any material that was “safe” and was either CertiPur certified (for polyfoam), or with any type of innerspring or latex (all latex will be certified as “safe” either by Oeko-Tex or Eco-Institut). There is more about the “safety” of the most common materials you will find in mattresses and different certifications that may be important to you in post #2 here along with post #2 here and post #6 here that should be helpful. I would avoid memory foam for children.
Post #2 here includes the better forum posts and topics about mattresses and children that should also be helpful and they also include links to some good quality/value options for children.
The better options and possibilities I’m aware of in the London and Southwest Ontario region is in post #8 here.
I’m not familiar with Zen Bedrooms but there are more than enough warning signs that show up with some quick research about them that I would be very careful in buying from them (and personally I would tend to avoid them or other companies that do most of their business with “coupon” sales based on fake retail prices and where you don’t know who you are really dealing with). There are a lot of red flags here.
The mattress you linked is made by Naturepedic (see here) who make mattresses using high quality materials, many of them organic. It has an innerspring and organic cotton padding and would certainly be a “safe” choice. It would also be suitable for children and for those where an organic certification was more important than just “safe” materials although it also may be in a higher budget range than you are comfortable with or compared to other good options that are also available to you.
Phoenix