This girl's dress looks like a miniature version of an adult's gown. The dress was probably made from another dress, perhaps from the girl's mother. Nevertheless, it is in remarkable condition considering that it had been worn by a five year old.
Click here to read more about this dress on the Corsets and Crinolines website.
Learn more about children's clothes here:
Jessamyn's Regency Costume Companion
Children's Costume History
Regency Children's Clothing: Day Wear and Play Wear
Changing Children's Fashion