Hi pblfive,
In most cases … the 'best" choice for a child is firmer than most adults would be comfortable with so I don’t think that firmness would be an issue.
If for some reason it is … then post #4 here has some of the better sources for toppers I’m aware of. For a younger child … I would lean towards either a latex topper in the 1-2" range in medium to medium firm that used latex that was certified for toxic ingredients or offgassing and VOC’s by a reputable certifying agency such as Oeko-Tex standard 100 (which would include all the Talalay latex you will encounter) or Eco-Institut (some Dunlop latex) or a wool topper that was manufactured without the use of harmful or harsh chemicals (either organic or made by some of the US manufacturers that use wool that is natural and “safe” even though it isn’t certified as organic). The linked topper post includes a link to several sources for wool toppers.
For a mattress protector … there is more about the different types in post #89 here but I would lean towards the waterproof / breathable type with a membrane that doesn’t contain vinyl or offgas VOC’s and again is either organic or Oeko-Tex certified (several of these are listed). With these types of mattress protectors it is particularly important to make sure you follow instructions with washing because they can lose their waterproof qualities or delaminate if you don’t wash or care for them according to the instructions and then you could easily be 'surprised" when there is an accident and you discover they are not as waterproof as you had hoped.
One other type of protector that may be worth considering is an organic wool “puddle pad” (a google search on “organic wool puddle pad” will bring up many options) which uses felted wool and is water resistant but these are not “waterproof” and larger accidents that aren’t cleaned up fairly quickly may soak through to the mattress.
The only way I know to do this would be to encase the mattress in clear non toxic plastic (no vinyl) of at least 5 mils thickness to prevent any offgassing or particulates from foam breakdown from coming through. This would be noisy and fairly uncomfortable. If you did this I would probably use a mattress pad or topper on top to isolate the feel of the plastic. Any other cover that breathed at all would still allow gasses and VOC’s to come through.
Hope this helps.
Phoenix