Officers in Indiana, US, say the youngster would be kept in the two-foot-square box for “extended periods of time”.
It was placed in the living room but the girl was rarely spoken to, police say.
“Nobody spoke up about a little girl in a box, even while we were in there, and started taking people out,” said county sheriff Jeff Richwine.
“It’s one of those deals, you go there and think, ‘Hopefully, this is not true. Nobody’s going to have a small girl in a box, that’s just not going to happen.’
“You’re thinking, ‘My God, who would do this?'”
Police found the girl asleep, curled up inside the box with the lid down. They said the box contained many dead bugs.
Those arrested include the girl’s father, Christopher Short, 25, his 42-year-old wife Donna and Patricia Meeks, 18.
They face neglect charges, while six others were charged with failing to report child abuse.
Police went to the house after a tip-off from Mrs Short’s ex-husband, who is in prison.
In court papers Mr and Mrs Short claimed the girl went into the plywood box voluntarily.
The girl is now in protective custody, and five other children also found in the home have been taken into state care.