Hi kercove,
Kevlar is the same material as they use in bulletproof vests and it’s a safe material yes (see post #13 here).
If you are leaning towards being as “safe as possible” then I think most people that are leaning in the same direction would probably be considering latex, innersprings, and natural fibers for a child’s mattress. A relatively thin layer of latex on an innerspring, an innerspring/natural fiber mattress, or about 6" of latex would all make good choices in a suitable firmness level. I would also lean towards having a wool quilted cover or a wool mattress pad because the temperature regulation and moisture regulating properties of natural wool would also make a good choice for a child IMO (see some of the benefits of wool here).
Polyfoam would also be “safe enough” for most people if it was CertiPur certified or was made in North America but it’s main benefit vs other materials would be cost not performance so this would be a budget choice more than a “better” choice.
If you use high quality and durable materials then when your son begins to mature and develop and reaches an age where he may need some additional softness and pressure relief then your mattress will still be in good enough condition that all you would need to do is add a topper rather than buy a whole new mattress.
Phoenix