There are a wide variety of fire sheild retardant products that you either mix with water and bug spray everything down or you add 8 to 12 oz of this miracle white powder to each gallon of paint. Small quanities go for $7 to $8 per gallon and buying in bulk is the way to go, you may easily spend $200 to $300 per 1000 SF covering things.
There are industrial products and then the next level down and everyone is using the next level down. Industrial product ratings are 5 times higher and so would cost about $5,000 to spray 3,000 SF. So you need to know your parts and products so the fire marshal does't demand the industrial rating stuff.
It is still an upsell for paint manufacturers because it does tend to settle in premixed materials and must be mixed at the time of use as an additive. It is then an upsell. Residential codes have the potential to change at some point and even require sprinkler systems in homes above 4800 SF and all homes should be 4800 SF but so far this gets held back over and over due to the extra cost.
Everything has a melting point or a flash point when exposed to open flame. Plastics melt and give off fumes more than transmit flames, which is bad but it is already an acceptable time period because most items are petroleum based or already have mineral or fish oil pressure impregnated as a mold release. Flash points on oils require open flames of several minutes to reach flashpoint that is usually around 550 degrees. So you spray this water based or latex mixed sheild on every thing.
No one is treating costumes as far as I know, only props and walls. There are quite a few threads on these forums to search that have the manufacturers listed like New York Fire shield and some haunt specific suppliers.
Another fabulous post from the U.S.Department of Wild Imaginings, now in spectaclar stereo, sponsored by the Adhesives and Sealants Council, suggesting ways to stick things together since the 1800s. Not fabulous in a gay way. Your results may vary. Illinois residents add 8% sales tax. These posts have been made by professional post makers, do not try this type of posting on your own without extensive training, lovely assistants and a trusty clown horn.