HELP! Daughter Is Reacting to the Elastic in Her Swimsuit!

It may be the Chlorine that is truly the problem... it's VERY toxic..
However, if you still choose to swim in chlorinated water, you could have her try a one piece that is all cotton (they are out there, but you have to search around) .... OR a cotton board short or something on the lines of it... and instead of a true bathing suit top, look for more of a workout top, now that might sound strange, but you CAN find those in all cotton... you d probably have to buy something like an XS so that it fits her, but hey, there are many cute ones out there... I have seen them with spaghetti straps and in some cases, they look more like swim suit tops than a workout top, so you see, no one would know the difference...
and when it comes to smaller sizes, they don't need all the elastic and extra padding, so you could very well find something that would work in this situation..

I would try the leotard. I would also see if she might have a latex allergy. The stretchy thread that is sews the arm and leg holes can contain rubber/ latex.

someone suggested vaseline as a protective barrier and i am going to strongly caution against it. while it may help as a barrier, it will intensify a sunburn. so please don't do this. you can try the leotard but if the reaction is from the chlorine reacting with the elastic there will still be an issue. i would seek out a sewist and have them make one with no elastic or have her swim in a rashguard and boys swim trunks (cut the elastic off the netting)
while i was sorking as a lifeguard i got a rash i thought was from the swimwear turned out to be severe heatrash that i could eliminate by staying cooler. (i dunked myself every 15 minutes to stay cooler we also had a 15 minute rotation so it was easy to keep track of time.)

I'm with you try the leotard!

Please take her to a dermatologist so they can help you figure out what is causing your daughter to break out.
I don't think it's from elastic/chlorine/formaldehyde.
I would not put her into the pool wearing a leotard just to see if her skin reacts.

Maybe apply Vaseline to the most sensitive areas as a protective barrier?

Just use a leo. They use a clear plastic stretchy material in the seams, it's not elastic.

Although I think if this was an ongoing issue that I would just let her wear a pair of smaller booty shorts with undies under them if she needs that security and a sports bra or something.

What did your doctor/allergist/dermatologist (not sure who you've seen) recommend with regards to formaldehyde with respect to clothing? Any advice?

My son doesn't have an allergy to formaldehyde, but he does get contact dermatitis so we were always told to wash his new clothes and bedding up to 3 times (the hotter the water the better) to remove that residue (they sometimes add formaldehyde to fabric to keep wrinkles out when it's new). I'm guessing the swimsuit has been washed? If not, maybe wash it a few times and see if that helps? ... not sure what else to tell you. I think your doctors may have seen this with their other patients and could better advise (especially an allergist). Ours gave us tons of advice for dealing with my son's skin.

He also gave us allergy meds and also an ointment (which our dermatologist ok'd as well) in case his contact dermatitis flared up. We still haven't identified everything, so we just give him allergy meds and use the cream if he gets hives or a rash from time to time. Takes care of it for us. An allergy may be a bit more severe .. so that's why I'd check with your doctor about the elastic. Otherwise your poor daughter may run into this from now on with swimming (hard to avoid). :(

Good luck - hopefully you can figure it out.