Hi softpick,
The mattress you are looking for is a little bit narrower than a standard twin size mattress (it’s actually a UK standard twin size and is also a size you will find as a hospital mattress or some institutional mattresses as well). While you will occasionally find mattresses in this size, you may need to have a mattress custom made in the size you are looking for.
You may have read this already but just in case you haven’t some of the better forum posts and topics about mattresses and children are listed in post #2 here and the information and mattress sources they include may be helpful.
That would depend on the type of mattress you are are looking for and your preference of materials. If you are comfortable with a polyfoam mattress and they can also provide a suitable cover for your foam layers then they would certainly be well worth considering and talking to. I would tend to avoid memory foam with children’s mattresses.
All foam will have some smell when it’s new but I wouldn’t think that theirs has any unusual odor compared to other US made polyfoam. I’ve never purchased foam from them however so I have no first hand experience and they would be a better source of information about the odor of any foam they sell. If the foam you buy is made in the US or is CertiPur certified if it’s not then for most people it would be “safe enough” in terms of harmful substances or VOC’s. For childrens mattresses that you only need to use for a couple of years I would say that 1.5 lb density polyfoam or better would be fine.
The tutorial post also links to a list of the members here that sell mattresses online and many of these would also be able to make a mattress in the size you need. There is also a list of lower budget options in post #4 here that would be well worth considering and talking to as well (some of which are also on the members list) and many of these would also be able to make a custom size as well.
Phoenix