I totally understand that you do not want to waste the investment you have already made in a mattress, and I think I might have good news for you.
First of all, you are at greater risk of exposure to flame retardants from other foam products than from mattresses.
- Sofa upholstery, foam insulation, the padding in high chairs or strollers all are made with petroleum foam treated with flame retardant chemicals.
- The flame retardants can come out of the foam as Semi Volatile Organic Compounds, and become part of your house dust.
The fire standard for mattresses is stricter than the one for upholstered goods, and the few seconds that flame retardant chemicals delay ignition are not enough to pass the mattress standard. Therefore most mattresses meet the fire standard by using a fire barrier cloth instead of flame retardant chemicals.
- I have written about this subject in a previous answer on GreenHomeGuide (here).
- The treatment usually is boric acid, silica, or phosphates.
- Occasionally this barrier might contain the the halogenated flame retardants we want to avoid, but the foam does not.
With that said, an organic and natural mattress topper will give your daughter an additional layer of protection between whatever is in her mattress, and the air that she is breathing for 8-10 hours during sleep.
Additionally I recommend that you vacuum her room a couple of times a week, with a a HEPA vacuum. The flame retardant chemicals have primarily been found to be present in house dust, and one of the biggest methods of exposure is consumption of this dust. This is especially true in toddlers, who put things that were on the floor into their mouths. I hate vacuuming, but this an important way to limit exposure to flame retardants.
Terra Publishing Company has published research (here) describing how we become exposed to flame retardants. For adults there is a strong dietary component; for children dust is a big factor.