Authorities were warned in February when a witness reported being sent one of the exploitation images by Justin via cellphone.
Investigators said the child's mother had noticed changes to her daughter's behaviour, as well as 'unexplained' injuries.
The FBI obtained search warrants and scoured the Crandall's home, finding more sexually explicit images and video.
With a trial just weeks away, Jessica Crandall, 27, have admitted both she and her husband had engaged in 'repeated sexual activities' involving the child. She faces up to 15 years behind bars.
Justin Crandall has also issued a guilty plea. He could serve up to 30 years in prison.
FBI court documents attribute Justin Crandall's behaviour, in part, to his meth addiction.
"Crandall admitted he touched the minor female child in a sexual manner while he was under the influence of methamphetamine," court documents read.
Jessica Crandall admitted she and her husband engaged in "repeated sexual activities" involving the child going back to December.
The pair, who will remain in jail, will receive their formal sententences early next year.