Merchandise that is potentially flammable such as apparel, is subject to flammability testing in order to confirm its acceptability for importation. Wearing apparel that is determined to be flammable, or that for which has not been tested to gauge its ability to ignite, may not be imported into the U.S. nor offered for sale here.
Simply put, it is banned.
Banned from importation, banned from sale here in the U.S., and banned from even the offer of sale.
Articles such as sturdy textile costumes*, including those which would be tied on, whether of a bib style or a waist-to-knee variety, are typically treated as articles of apparel and clothing accessories by US Customs as well as under the Flammable Fabrics Act (FFA). (*This type of costume is not to be confused with a flimsy variety which would be classified under Chapter 95 HTSUS)
Since they are considered apparel, this merchandise is subject to the flammability regulations set forth in 16 CFR Part 1610.
All textile fabrics intended, or sold for use in, wearing apparel, and all such fabrics contained in articles of wearing apparel, are subject to the requirements of the FFA, which are enforced by the Consumer Product Safety Commission (“CPSC”). 
Under FFA, when a fabric (or any uncovered or exposed part of it) is so highly flammable as to be dangerous when worn by individuals, and where it exhibits a rapid and intense burning when tested under the testing conditions set forth in Subpart A of 16 CFR Part 1610, it is prohibited from importation, and banned for sale or offer for sale here in the U.S.
The purpose of the FFA testing requirements is to prohibit the use of any dangerously flammable clothing textiles in order to reduce the danger of injury and loss of life.  With regards to general labeling requirements for adult wearing apparel, information about the fiber content, country of origin and RN number information should be included.
While a label is not required to be sewn onto the apparel indicating CPSC compliance, or non-compliance, a General Certificate of Conformity is required with importations of apparel that declares that compliance with CPSC enforced laws – including FFA - has been met. This certificate must also be kept for both recordkeeping purposes as well as to furnish to retailers and/or CPSC upon request.
CPSC is the enforcement agency for violations of the FFA and under the current regulations, the maximum penalty amount for a known violation is $100,000, with a maximum penalty for any related series of violations being $15,000,000.
With penalties this extreme, it is imperative that importers obtain the requisite compliance advice pre-importation rather than to cut corners and be put out of business later.
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 The requirements of 16 CFR §1610.1(e) state that “[t]he requirements of this part 1610 shall apply to textile fabric or related material in a form or state ready for use in an article of wearing apparel, including garments and costumes finished for consumer use.”
 16 CFR §1610.1.
 While California’s Proposition 65, which deals with levels of chemicals, requires a label when a product is not in compliance, federal regulations do not require such labeling under the flammability rules as the product is merely banned from importation and/or sale.
 CPSIA Sec. 217(a)(4).