I would not buy flame-retardant batting for a baby quilt.
Flame-retardant fabrics are mandated for the sale of loose-fitting children's sleepwear because a single layer of cotton, as in a child's nightgown or loose PJs, is exposed to air on both sides, making it highly combustible (engulfing a child in flames within 10 seconds of ignition). A quilt is not highly combustible in this way because of its thickness.
Blankets, for example, do not ignite in the way that draperies do in movie scenes. The draperies are exposed to air on both sides, plus they are hanging, so flames can quickly climb up them. A mattress, in contrast, may smolder for hours because of lack of air to feed the fire.