Prudence Cruff is the daughter of the inappropriately-named Goodman and Goodwife Cruff. This revolting pair treat their daughter like dirt, constantly abusing her both physically and mentally.
When Kit first meets Prudence abroad the Dolphin the little girl comes across as a meek, fragile creature, someone who wouldn't say boo to a goose. After Kit dives into the sea to save Prudence's doll it's notable that the little girl's alleged mother isn't exactly bursting with gratitude. This is because she's thoroughly embarrassed at what she sees as her stupid daughter.
That Mrs. Cruff is so quick to describe her own daughter in such harsh, unflattering terms gives you some idea of how the poor child is treated at home. It also gives us a possible reason for why Prudence's parents don't send her to school, or indeed give her any kind of education. They clearly think she's so stupid that it would be a complete waste of time.
However, as Kit herself subsequently discovers, Prudence is far from being stupid. She's just a quiet child who needs lots of support and encouragement. Although she doesn't get it from her parents she most certainly gets it from Kit, who teaches Prudence to read and write.