To get grass stains out of clothes you can buy a pre wash stain removal treatment that you can apply to the grass stains and let it sit for awhile.
Then wash the clothes like normal with your normal laundry detergent and the grass stains should be removed.
You can also mix one part of white vinegar to two parts of water and spray the solution onto the grass stains and then use a scrub brush or toothbrush to scrub the grass stain out.
Then wash it again in the washing machine.
Sometimes just soaking the clothes with the grass stains in the washing machine with some vinegar and detergent for a few hours or overnight can help get the grass stains out of the clothes depending on how long the grass stains have been there.
Sometimes you might have scrub the grass stains out and maybe even use some hydrogen peroxide on the grass stains which sometimes helps as well.