Guinea Keets got into some strayfoam, will grit help?

I have a pair of blue guinea keets that are starting to fledge really well. They are turning a nice Coral blue, but last night they managed to find n peck at some stryafoam. So now i'm panicing cause i feel like it's going to kill them and i was just starting to get them really tame. They have been on turkey starter crumbles so they haven't had grit, if I feed them grit will it save them. What type grit do u feed can i feed them some sand from outside. Help Guinea or Chicken people HELP!