The girls were between the ages of 7 and 11, and the offending took place over two years, from 2008 to 2010. Parkinson was in his mid-30s at the time and was living with his wife and daughter.
The offences against two teenage boys, aged 14 and 15, occurred in 2015.
Parkinson provided the boys with alcohol, showed them pornography and let them play on his computers and gaming consoles. After one particular offence, Parkinson paid one of the boys $100.
In court, the four victims revealed how the offending had damaged them.
One of the girls said Parkinson had ruined her teenage years, had impacted on her relationship with her family and had left her unable to trust men.
One boy suffered stress and had trouble sleeping, while the other was nervous going to other people's houses and had lost control of his behaviour.
At Friday's sentencing, Justice Helen Cull told Parkinson: "It is clear that your offending has had a profound impact on all of the victims and their families.
"The statements indicate that each of the victims have, in his or her own way, been psychologically and emotionally damaged by their experiences."
A psychological report, prepared for the judge, revealed that Parkinson had, himself, been a victim of sexual abuse as a teenager, while the judge referred to his mental health issues and problems with alcohol and drugs.
A pre-sentence report said he was at a high risk of re-offending.
"It may be now, that the impact of your offending has become tangible, you are comprehending the serious consequences of your actions," Justice Cull said
"You have expressed the desire to leave prison and never offend again. You reported wanting to be able to tell others about the impact of sexual offending against children, now that you have witnessed the consequences of your actions first hand."
In sentencing Parkinson to six years - with a minimum three years to be served - Justice Cull gave weight to the serious harm done to the victims; their age and vulnerability; the breach of trust; repeated offending; and pre-meditation.
She also called for Parkinson to receive "intensive treatment" while in prison.
"This will likely reduce your prospects of re-offending if your willingness to engage in rehabilitation is translated into action. This needs to be a priority of your sentence.
"I want to see that at the end of your sentence you have received the treatment you desperately need."
