Hi earts,
It should be fine for a child that is under their suggested weight range.
I would keep the coir since it will provide firmer support for the pocket coil.
It would depend to some degree on the thickness of the fiber. It will provide a slightly softer and more “relaxed” surface feel vs sleeping directly on the latex and it would provide some extra breathability as well so it may be a little more temperature neutral but the fiber will also tend to pack down and break down over time and to some degree will lessen the ability of the latex to contour to the shape of the body. There is more about the pros and cons of a quilted cover vs a more stretchy knit cover in post #12 here. I would treat the choice as a preference.
The main difference between an adult mattress and a child’s mattress is that the adult mattress would most likely have a thicker layer of latex and would be more suitable for higher body weights in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences). There certainly wouldn’t be any harm in lying on your son’s mattress for story time even though it may not be the most “comfortable” or suitable mattress for you to sleep on over the entire course of the night.
An adult mattress that is in a suitable firmness range for younger children would also be suitable for your son but there would really be no meaningful benefit to spending more if you only plan to use the mattress for 5 - 10 years and if he does need some additional softness and pressure relief as he gets older and develops more adult proportions and weight you can just add a topper to the child’s mattress.
Phoenix