The East Rand Car Salesman was sentenced at the Johannesburg High Court.
Judge Cassim Moosa said the accused has pleaded guilty of rape, which is a serious offence with a minimum sentence of life imprisonment. He said there was increasing pressure on the court to give harsher sentences to individuals who are convicted of rape.
Knoop committed the sexual offences between 2011 to 2015, in the East of Johannesburg and in Mpumalanga province.
"Children are usually abused by those who believe they will get away with it, and they usually do," Judge Moosa said.
The mothers of two of the girls who were assaulted by Knoop were in the public gallery, as well as Knoop's brother.
The 40-year-old was caught after he used a chat website to distribute child pornography. Homelands Security in the United States of America (USA) notified South African authorities, which then located and identified him via the cellphone network account he was using.
On Friday Knoop said that while in prison he found God and had been rehabilitated. He told the women he dated that he did not date them because they had children and that he was sorry for what he did.
Judge Moosa noted that: "There are no substantial and aggravating circumstances which will call for deviation from the minimum prescribed sentences".
"A sentence higher than the prescribed minimum would be appropriate."
Judge Moosa said that a strong message needed to be sent to like minded individuals, that South African courts will not condemn such actions. Knoop was ordered to take part in anger management and sexual offender classes to assist with his rehabilitation.
The mother of the 12-year-old that was assaulted by Knoop said: "She's receiving trauma counselling and is still having suicidal tendencies. His testimony was a joke. He was always saying I shouldn't drink or smoke and dress a certain way."
The 28-year-old mother of the second child said that Knoop arranged a private investigator to look through her phone messages and recorded her calls as he thought she was cheating.
The accused was deprived access to his biological daughter after his wife found him viewing child pornography.
"There are numerous other children. We were the only mothers who were willing to come forward," one of the woman said.
The women said in their evidence that Knoop was a gentleman, kind and a provider when they were in a relationship with him, and his actions came as a shock to them.
National Prosecution Authority (NPA) spokesperson Phindi Louw said: "We entered the plea with the view with a sentence with reasonable prospect. We are glad that the court has confirmed that the plea was just".