Hi cristalyte,
You’ve may have seen this but just in case post #2 here includes links to some of the better forum threads about children and mattresses in the forum.
My guess is that a three inch topper would probably be OK if there were no better alternatives … at least for a while … but it’s outside of my experience so I don’t know how long they would be comfortable on it. I would just use your best judgement about when it appears they are no longer sleeping well on it and may need some extra layers on top.
That would depend on how far apart the slats were. A latex “mattress” needs to be evenly supported by a firm non flexing and evenly supportive surface and the slats should be no further than 3" apart (and preferably less, especially with such a thin mattress).
A piece of MDF or plywood (I don’t think I would use particle board) would work for support but it would also lack ventilation so it would be a little more risky than a support surface that had better ventilation. You can read a little more about this in post #10 here. If there were no other risk factors that would increase the risk of mold or mildew or other ventilation or mousture issues then it would probably be OK as well.
Generally no but in an apples to apples choice of the same ILD I’d probably lean towards Dunlop in a layer that thin because it gets firmer faster with compression and there may be less risk of going through the topper and feeling the support surface underneath it.
The initial offgassing would be over but foam also breaks down and becomes part of the household dust as it ages. While most people would say it’s fine and wouldn’t be concerned, there is really no way to answer this with any certainty because each person may have a different answer to “how safe is safe enough for me”. Post #2 here has more information and links to some good resources about a very controversial subject that has no clear answers but lots of strongly held opinions on both sides of the argument.
Phoenix