Hi emm14.
Welcome back to the Mattress Underground forum all the way from 2012 and Congrats on your daughter’s Arizona Premium Mattress 
I had your post on my board since yesterday but some urgent business pulled me away and did not manage to end it. Great to see that your post got so much attention from TMU Expert and Trusted members. All great advice! This is a good case study for others that have simmilar questions, so I’ll add my reply as well.
We recently purchased the Arizona Premium kid’s latex mattress with a specification of 3" of Medium ILD #32 and 3" of Firm ILD #36. One of us will occasionally sleep with her a night so wanted something that could also accommodate our preference for firm.
Our daughter is fine on the mattress given she’s still very light (<35 pounds), however, we still find it a bit on the softer side and somewhat thin (almost feels as if we can feel the bed slats).
Making the mattress a little firmer to accommodate you and your partner won’t create any alignment or support issues for your daughter. Children are very adaptable and they do very well with firmer surfaces which are needed to support their epiphyseal plate and postural formation. You can read more about firmness recommendations for children in different age groups in posts #3 and #13 here.
We’re trying to determine what would be recommended to make the mattress feel firmer:
Adding a firm polyfoam or latex base layer - would a 2" or 3" firm support layer help in this case?
Adding more bed slats so there is less spacing - is there recommended spacing between slats?
The “lack of firmness” that you feel with a 6" mattress may be a combination of factors:
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Your particular stats (BMI, sleeping position, and PPP (Posture and alignment, Pressure relief, and Personal preferences) play an important role in how the mattress feels to you. You did not mention anything about your stats but as an example… low BMI people can do very well with a 6" thick mattress and feel adequately supported.
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Too large of a gap between the slats (4" is more than what is recommended for a latex mattress) Less than 3" is recommended for latex but I’d personally use less than 2.5" gap. You can read more about best recommendations for latex or all foam mattresses starting with Post #1 here
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The slat area in contact with the mattress is also a factor. A larger slat area will support the latex more uniformly.
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I’d check that the slats do not flex under the bed when one of the parents is sleeping along. (Children’s beds may use thinner slats, and soft or flexible wood) You can do a few things:
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Measure the distance between the wood and the floor with sleepers on it and then with no sleepers on it. If the measurement is the same then probably there is no flex … and you can eliminate this as a variable.
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I’d put the mattress on the floor and sleep on it for a while to help determine if some of the “firmness” loss is caused by 1) the slat flex and 2) a larger gap between the slats. Assuming that you did the measuring and eliminated slats flex as a cause … if you feel more supported and you don’t “feel the slats” then the cause (at least partially) is that the latex being a point elastic material will stretch and occupy more of the gap between the slats. The larger gap will allow more “bunching in” of the rubber material and “stealing” some of the layer thickness. This creates a non-uniform support layer that may result in the symptoms you describe.
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Another thing to try while having the mattress on the floor is to try to flip the mattress and see how it feels to you with the firm layer on top.
Either way, I’d start adding more bed slats to provide enough slat surface area (and not void the warranty of the mattress). Once you find out how it feels when this is addressed you can determine if adding or changing a layer is appropriate.
Once you have the correct slat gap you may want to try to put the 3" of Firm on top of the 3" medium by flipping the mattress to see how it feels. This “reverse layering” the firm layer on top of a medium layer will reduce how far you sink through the upper layer to feel the effects of the layers below which can work well in certain cases.
Phoenix
Note: I have changed the subject line to better reflect the topic in the discussion from " Recommendation for firmer mattress" to “Recommendation for firming child-parent mattress”