I actually seek out non-flame resistant pajamas for my daughter. The chemicals used to make the fabric flame resistant will leach out and be absorbed through your childs skin.
For years TRIS has been used in Pajamas, until it was banned in 1977 for causing cancer.
After that PBDE's were the retardant of choice for textile applications. That stuff accumulates in the food chain and has been implicated in effects on brain development and possible environmental effects, especially in large predators (whales for example). Through direct exposure and the food chain they also accumulate in human fatty tissue and are excreted in breast milk...mother with high levels of PBDE's will pass on the chemicals to their children, long after the chemicals have been banned (they have been phased out in 2004)
Nowadays most fabrics are treated with PROBAN. The chemicals to to produce it are linked to genetic defects, damage to liver, skin and nervous systems. It might also cause cancer.
If your child wears a tight fitting sleeping garment and you practice common sense fire prevention you should be fine without exposing your child to potentially dangerous chemicals.