feed them on crushed flake and small live foods, when they're about 2 weeks old you can tie a green mussel in the tank and watch them attack it as well as the above foods
Guppy fry require somewhat warmer temps and more feedings then their parents to ensure good healthy developement. A variety foods(especially rich foods like bloodworms) are best for nutrition. I feed my fry finely crushed flakes, bloodworms, and tubifex worms. Since you need to feed them more that means also more water changes.
Good luck with the fry