Two women, who can only be identified as G and T, pleaded guilty in the Launceston Supreme Court to sexual crimes against children including having maintained a sexual relationship with a young person and having involved a young person under the age of 18 years in the production of child exploitation material.

T pleaded guilty to further offences of aggravated sexual assaults and indecent acts with young persons, as well as summary offences of having made a bestiality product and drug use.

The third co-accused, W, a man who is in custody, has pleaded guilty to his charges and will face a plea hearing in December.

Some of the child victims have said nothing to police and the crimes which were committed against them were only corroborated by what other children had seen.

The crimes were uncovered last year when a digital memory card of child exploitation material involving some of the children was accidentally found and the police were notified.

G was in a relationship with W for about 18 years and they had four children together, Y, X, and two younger children.

Later, the couple and their children started to live with T and her three children, Z, V and W.

All of the children, except G's two youngest children, were sexually abused during the next three years.

The children were given methylamphetamine at times to ensure their compliance.

During the women's pleas in mitigation, their defence lawyers said that both co-accused were of low intelligence and had been victims of incest during their childhoods.

Counsel said that the women found themselves in an abusive relationship with the dominant W who was the primary instigator of the sexual abuse.

Justice Robert Pearce jailed G for five years with a non-parole period of three years.

He also jailed T for six years with a non-parole period of three years and six months.