Hi sab,
All of these would be good quality/value choices and they all use high quality materials and have good value.
The main differences between them is that Spindle uses two 3" layers of 100% natural Dunlop that is made by Mountaintop foam with a continuous pour process.
Flexus uses 100% natural Dunlop made in a mold.
SleepEZ gives you the choice between either 100% natural Dunlop made in a mold or blended Talalay.
All of them have a cotton and wool quilted cover.
I would also consider a twin XL instead of a twin if your son is likely to be taller so he doesn’t outgrow the mattress when he starts to get taller.
All of these also provide a choice of firmness levels and you can either add a topper when the time comes that he may need it or you could order a thicker cover and add an additional layers inside the mattress.
The My Green Mattress 2 sided organic cotton would also make a great choice (either with or without the latex which would make it softer) but this is a completely different design and the choice between them would be a preference choice based on which materials you would prefer.
The zoning in the Spindle is “mild” meaning that the differences between the zones would be fairly small and it isn’t likely to be an issue for a child or for those that are a normal height but you can talk with Neal about this because he will have more experience with this mattress than anyone else.
When you are down to finalists that are between “good and good” where any one of them would make a great quality/value choice and there are no clear winners between them … then a final choice (see post #2 here) that would be “best for you” would really come down to “best judgement” based on how you feel about your more detailed conversations with each of them and all the objective, subjective, and intangible criteria that are most important to you.
Any one of them would make a great choice for a child.
Phoenix