Mold is a fungus that thrives in areas with the right environmental requirements: moisture, darkness and warmth. Besides being unsightly and smelling rancid, mould has been linked to a number of different health and respiratory problems. This is especially concerning for children, as mould may wreak more havoc upon their younger immune systems than upon that of an adult. Mold on a car seat is a serious problem and should be handled immediately.
Remove the car seat from the car. Mold can spread very easily and it doesn't do much good to put the clean car seat back into a mouldy car.
Use a soft to medium bristle brush to scrape away as much of the mould as possible.
Mix a solution of hydrogen peroxide and water in equal parts in a spray bottle.
Spray enough solution onto the area to saturate it and allow it to soak for 5 to 10 minutes.
Use a soft clean damp cloth dipped in warm soapy water to scrub the stain clean and use a clean damp cloth soaked in clean warm water to "rinse" the area.
Allow the car seat to air dry, outside, in the sunshine. Sunlight is great for killing mould spores.
Store the car seat in a cool dry place when not in use to prevent mould in the future. Test an indiscreet area of the car with the solution for colour fastness. Vinegar and colour-safe bleach are good alternatives to hydrogen peroxide.
Ventilate the area well when dealing with mould, and be sure to wear gloves and a mask.