Can anyone suggest?

The only thing I can think of is to remove candle wax but it may help to get the worst out of your carseat. First scrape of as much of the crayon as you can. Use papertowels or tissues and layer a few at a time on top of the mark then use your iron (start of on a cool setting so as not to melt the upholstery and slowly turn up the heat if needed) and run the iron over the top of the tissues. Once crayon is soaking into tissues replace with clean tissues and carry on until the worst is gone. Once done you could try a carpet stain remover or a drycleaning fluid to remove the remaining stain. Best of luck.

Make sure Kafeel is locked inside the car, before you follow his/her instruction.

I suggest you check with car wash companies like Teyseer service centre (Opposite Airport), who has car upholstery cleaning.