Hi susiebmck.
While the mattress uses great quality/value materials in the case of being used by a growing child, I would tend to choose a firmer top layer mainly because children tend to do better on firmer and more supportive mattresses. As your daughter moved into preschooler/early childhood stage it is best to adjust her sleeping surface towards the medium, medium firm. While growing children can adapt to almost anything you’d still want to lean toward a bit of a “firmer” especially with children sleeping in multiple positions over the course of the night.
Sleep On Latex has several good options that can be adapted to suit growing children. If you wish to stay with the thicker 9" mattress then I’d probably move the 2" Dunlop uppermost layer firmness one notch up with something in the range 28-32(medium) to ensure good posture and support during the forming of her epiphyseal plates. The upper 2" @ 20 ILD layer in the 9" firm version may still be too soft to keep your daughter’s proper alignment …I’d recommend calling SOL to ask if they would be willing to replace the 2" soft with one of the 2" medium (30 ILD) topper. Pure Green has a zippered cover and one of the advantages of having a component style system is that later when she develops more adult proportions and her comfort needs change you can easily change the top layer to something softer for extra cushioning.
Because generally, children won’t really need more than about 6" of latex another option you may wish to consider is the 7" Pure Green version and when your daughter grows older you can add a topper for some additional thickness or softness or even add another layer inside a new cover.
Either way, I’d make sure to call Sleep On Latex to discuss all your options. As you probably are aware, they are a Trusted member of our site. which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of quality or their products, value, service, knowledge, and transparency. You may also wish to peruse post #2 here and Post #2 in this very topic which includes links to most of the better forum posts and topics about mattresses and children.
Phoenix