Hi kapy3,
I have learned over many years that what people feel on a mattress (or under a mattress) can vary widely and would all be “normal” even if it’s different between people but you certainly wouldn’t be the only one to feel what you are feeling. I’m not sure if you are feeling is the actual outline of the slats themselves or just their firmness and rigidity under the mattress but what you feel under a mattress will be a function of the firmness of the latex, the thickness of the latex, the concentration of weight in a particular area of the mattress (which is much higher in a sitting than a lying position), and the specifics of the slat dimensions and the distance between them.
Latex mattresses generally don’t have (or need) edge support for sleeping (see post #3 here) but there is an “edge effect” with foam materials because if the edges are compressed they are only being “held back” by the material on one side of where you are sitting (behind you) and not the other and because of it’s “point elasticity” each part of the mattress can compress more independently from other areas. This means that with the more concentrated weight of sitting in combination with sitting on the edge you will tend to sink in more deeply and feel much more of what is underneath the mattress than you would if you are lying down on the mattress, particularly if you sit close to the edges. This would be more noticeable with softer or thinner layers than it would with thicker or firmer layers which would isolate you more from the support system under the mattress but even a firmer latex mattress can feel softer and you will sink in more deeply if you sit right on the edge of it.
There are a few things you can do that may help and while none of them will affect how deeply you sink into the edge of the mattress … they can reduce the effect of what you may be feeling under the mattress.
First I would confirm that what you are feeling is the slats themselves rather than just feeling a “hard” and non compressing layer under the mattress. You could test this by putting some plywood under the area where you are sitting to see if you feel just the firmness of the plywood or whether you were feeling the actual shape of the slats themselves (which would be less likely with 1x4’s with only 1.75" gaps).
If this seems to help then it wouldn’t be ideal to put plywood under the mattress because it doesn’t breathe (see post #10 here) but some other more breathable options would include adding an inch or two of ultra polyfoam in between the mattress and foundation or adding a rubberized coir bed rug like this or even something like the vinyl lattice here that would all provide additional support in the gaps between the slats.
You could also sit a little bit further back on the mattress so that your weight was distributed over a larger surface area and wasn’t quite as close to the edge and see if that was enough to make a difference as well or it could be something that you will just get used to because while it can be an inconvenience, it’s not something that would concern me as far as your daughter sleeping on the mattress.
Phoenix