Michael David McCauley, 41, of Cwmbran, denied abusing children as young as six but was convicted of a string of offences and jailed for 15 years at Cardiff Crown Court in January.
The case was sent to the Court of Appeal on Wednesday, where the term was upped to 20 years.
Lord Justice Treacy described it as a "serious catalogue of offending".
"The result of the offending, certainly in the first girl's case, amounts to serious psychological harm," he said.
"What took place was planned and involved grooming."
McCauley was found guilty of six rapes and seven sexual assaults.
He also admitted having child abuse images, which police found stored on his computer, visiting sites and discussing paedophilia online, the court heard.
Lord Justice Treacy added: "We are satisfied that the overall sentence passed was unduly lenient."
A child rapist jailed for grooming young girls for sex has had his "unduly lenient" sentence increased.