I don't see why not, just make sure it doesn't have any seeds. I give my baby chicks lots of 'fresh treats' and have never had a problem. I've never had guineas but I hope I can get some this summer just make sure to give them a little grit and let them dig in!
Another treat I'm sure they'd love (my chicks sure did!) are small/x-small crickets from a pet store. Usually pretty cheap (around 50 for .25 here) and they go nuts for them! If you can stomach giving them live bugs that is
I'd just toss a couple in the brooder and watch the mayhem begin lol, small meal worms also work great too!