Grass stains on kid’s clothing can be permanent if they are not properly treated when they are relatively fresh. The sooner you see the stain the easier it will be to deal with. If you know the clothing is stained then when the child removes it you can spray it with any number of commercial stain treatments and this should remedy the situation. Most of the commercial stain removers will have you wash the clothing within three days of treatment.
Kid’s clothing can get put in their bedroom floor, or kicked under the bed, or otherwise placed so that the mother does not see the grass stain until it has started to adhere to the fibers of the material. You can take baking soda like you cook with, or keep in the fridge to make it smell fresh, and make a thin paste of it by adding a small amount of hydrogen peroxide. Using a soft bristled toothbrush scrub the stain with the baking soda paste. Most of the time this will lift the stain off the clothing.
Toothpaste can be applied to a stain that has gone unnoticed in the same manner that you apply the baking soda and peroxide solution. Use a soft bristled tooth brush and scrub at the stain. Rinse with cold water to get the tooth paste off of the clothing before tossing it into the washing machine.
Salt is often capable of getting the stains out of clothing. Wet the area where the stain is and apply a generous amount of table salt. Rub the cloth together so the salt can rub against the stain. Rinse with cold water and then wash as you normally would. The salt may cause other colors on the item to bleed so do not use warm water, only cold water.