To protect children from burns, these rules require that children’s sleepwear must be flame resistant and self-extinguish if a flame from a candle, match, lighter or a similar item causes it to catch fire. The rules cover all children’s sleepwear above size 9 months and up to size 14 and require that:

(1) the fabric and garments must pass certain flammability tests; or

(2) be "tight fitting" as defined by specified dimensions.

The regulations are published in the Code of Federal Regulations at 16 CFR Part 1615 for children’s sleepwear sized above 9 months and up to 6X, and at 16 CFR Part 1616 for children’s sleepwear sized 7 through 14. The two rules contain basically the same requirements, with the main difference being the sizes of the garments covered by each.

https://www.cpsc.gov/Business--Manufacturing/Business-Education/Business-Guidance/Childrens-Sleepwear-Regulations