Hi CeruleanBlue,
Post #2 here includes links to the better threads in the forum about mattresses and children including some good information about firmness choices and sources.
Children need a firmer mattress than most adults for best alignment so I would lean towards a firmness choice that was closer to what they needed than what you are likely to be more comfortable with. At the very least I would choose a firmness level that was the very firmest that you would be comfortable with.
This depends on the quality of the materials and the other durability factors that are mentioned in post #4 here and the post it links to but if you buy a mattress that uses very durable materials like latex in firmness levels that aren’t too soft then it could certainly last that long yes (you can see a video here of a thinner and firmer single layer latex mattress here that is over 40 years old).
My answer to almost all “should I” types of questions is that it would be up to you and would depend on the specifics and circumstances and on your personal preferences. In most cases I can’t answer these because they are a matter of preferences. A shikibed can certainly make a good firm choice for children although it may not be as comfortable as they get older and as the cotton compresses and becomes firmer or if they become side sleepers (which is much more common in western culture than it is in eastern cultures where the majority of the population are back sleepers). Of course you can always add a topper to it if necessary as well. It’s advantages are firmness and durability (it won’t break down although it will get firmer over time as the cotton compresses) and breathability and ventilation. It would take more care and maintenance though to make sure it has a chance to ventilate and release any moisture buildup that may happen with the cotton.
The Ecohaven is made from a Dunlop latex core and a 2" Talalay latex topper with wool qulting so it could easily last 10 years or longer yes.
That would certainly be an “ouch” and would be significantly more than you would need to spend IMO. I would tend to avoid brands like Natura this time around though because they are good quality but not great value compared to many other mattresses that use the same or similar materials and sell for less. They may also not be as widely available anyway since they have been dropped by a lot of retailers since their bankruptcy and purchase by Spring Air / Sommex.
Online purchases can be a good choice for children where it’s easier to determine what they need (medium firm to firm and generally thinner mattresses because of their lower body weight). If you let me know the city or postal code where you live though I may be aware of some local options that are within reasonable driving distance.
Phoenix