Hi occomposter,
There is a great deal of information in post #2 here and the topics it links to about mattresses and children including a link to some general firmness guidelines for children in post #2 here. It also includes a number of links to the better forum posts and topics about mattresses and children as well which have more information about many good options which may be well worth considering.
3" may be too thin and I would suggest 6" of latex in either one or two layers and then you can add a softer topper when they get older and develop more adult proportions if and when they need a softer comfort layer in their “sleeping system”.
95 kg/m3 is in an extra firm range and is firmer than I would suggest for a top layer for a 3 year old toddler. It would be OK as a 3" base layer with a softer 3" layer on top of it but even there it may be firmer than you really need (depending on their size and body type as they reach their full adult weight and proportions).
A wool quilted cover can also be a good idea because of wool’s natural ability to regulate temperature. There is also some good evidence that sleeping on wool can slow down the heart rate and lead to deeper sleep. This is probably from a combination of its ability to control moisture and humidity and temperature.
Phoenix