Hi Lizsky,
In “theory” a single layer may be a little more durable because layer combinations “act” a little bit softer and more independently and softness (and how deeply you sink into the comfort layers of a mattress) is a secondary factor in terms of durability (see post #2 here about the main factors involved in durability) but in practical terms if both of the separate layers were both the same ILD as the single layer then they would be closely equivalent in terms of durability and for me durability would not be a concern with either one in a latex mattress … especially with a younger child. More layers can also provide more options to customize the mattress (either initially or down the road as needs and preferences change) so for example you could use a top layer that was the same ILD as a single layer for the “feel” of it and then use a higher ILD base layer for firmer support. Both have their pros and cons in other words including any price differences between two mattresses and it would really depend on which one overall was the best match for your personal value equation because you would be making a final choice between “good and good”.
Phoenix