I don't have experience with grass specifically yet (my son just turned 2 and my girl is a girly girl). But, I would NOT dry them until you are sure the stain is out. Heat does set stains, but it is possible to remove some stains even after they have been dried in the dryer.
Here's a trick that I use- saturate the stain with Spray & Wash or Shout and then sprinkle with OxyClean- the Versatile kind, not the one with bleach. Let that sit for about an hour and then wash. Let it air dry and check to see if the stain is gone. Then if needed, you can repeat the steps.
This process has saved many clothes that I thought were ruined, including a hand-sewn heirloom-quality dress my daughter wore when she was a flower girl at my cousin's wedding.
On another note, OxyClean is great at taking the sour smell out of clothes. My son is a big drooler (always has been)- he can wet the front of his shirt in an hour or two. Anyway, adding a bit of OxyClean or soaking the really bad shirts always takes the smell out.