We bought a 6" all-latex mattress (just a slab of latex with a very light cotton cover – no quilted mattress cover) that turns out to be too soft for our daughter. I think it was medium. Would it help to get an extra firm 3" latex mattress to put under (or on top?) Or is there some other way to make this work better for her? Thanks for any advice!
Hi beanish,
Welcome to the Mattress Forum! 
The layers closest to the seeping surface will have the most dramatic impact upon comfort, so a 3" “extra firm” latex topper should create a harder surface comfort for your daughter. If the issue of being “too soft” was more of a deep support issue, you could try the 3" “extra firm” latex piece beneath the 6" latex mattress and see if that is preferable.
Phoenix
Great, thanks so much for the advice!
Hi beanish,
You’re welcome!
Phoenix