Hi DrPowell,
Welcome to the Mattress Forum! 
Congratulations!
The Sleep on Latex 7" model uses good quality and durable materials, and as you may be aware Sleep on Latex is a site member here, which means that I think highly of them and their products. Whether or not this product would be appropriate for you and your husband would of course come down to your own personal testing, but as you mentioned the do have a 100 day return policy.
A firmer latex mattress for children is generally recommended because their growing but less developed bodies need less contouring and firmer support. Starting in the upper 20s for an ILD into the 30s is usually an acceptable range to meet the recommendations of experts. Post #2 here has links to the better threads in the forum about mattresses and children that will be helpful, and also has some good sources for children’s mattresses. You can look at this thread from someone who was also looking for a crib mattress that may be helpful. Here’s another good post with some crib mattress references, as well as some good sources in post #2 here.
For custom sizes, with custom covers, I would use the manufacturers and suppliers listed in the component post here under the latex section as a good starting point. DIY Natural Bedding is on that list and they do use quality componentry.
I know this is a lot of information, but hopefully it expands your options to meet your specific requirements.
Phoenix