Hi pracies,
Post #2 here has a list of forum threads that concern mattresses and children that include many links to some very good quality/value mattresses.
I agree with you that Savvy Rest is costly but I also think that for 2" of latex, Earthsake is a costly choice as well.
Dust mites need moisture and if there is wool quilting in the mattress then this will keep the microclimate dry and inhospitable to dust mites although of course you can never completely avoid them. Latex is also the most breathable of the foams so this can also help with dust mites.
For a young light child it probably would be OK although i would tend towards a little thicker … especially at the price.
I don’t think there would be any way to quantify this but with latex and a wool quilting it’s not something I would be concerned about. Dust mites need a warm humid environment for food and water and wool is great at keeping the humidity at bay and regulating temperature. It stores moisture inside the fiber while the outside is still dry and then gradually releases it. I think that latex with a cotton cover quilted with wool is a good choice for a child. Post #2 here has a little more about dust mites.
You will find many better quality/value options in the threads I linked 
Phoenix