When a broody hen hatches her chicks one of the first thing she does is take them to a spot where they can peck at the ground. They get grit for their gizzard so they can grind stuff up that needs grinding, bits of stuff like vegetation in various stages of decay for nutrition, and just plain dirt for minerals. This is usually within hours of bringing them off the nest. She will take them to water and if you have chicken feed around she might take them to that first. It usually does not take her long to get them onto dirt though.
I'm not a broody hen. In the brooder I give them water and chicken feed for the first couple of days. Then when they have learned what chicken feed is I take dirt from the run and offer them that. It usually goes pretty fast. At 5 days old I would have already started mine on dirt.
A fly would be OK, it is soft and really doesn't need to be ground up. But after thy have had grit I would not hesitate to give them stuff they need to grind up. A broody hen has no qualms about doing that. Just make sure the chick feed is most of what they eat so they get a balanced diet.
