William MacKay, 70, had denied the abuse, which took place in Ross-shire between 1986 and 1994.
He was cleared of rape after a trial at Glasgow Sheriff Court but convicted of three counts of sexual abuse and jailed for three-and-a-half years.
The court heard his victim experienced flashbacks and had dream therapy.
That unlocked memories of the abuse of her and another girl that MacKay used to babysit for.
One of his victims was only seven when her ordeal began.
Defence QC Frances McMenamin said that MacKay, who is in poor physical health and suffers from short-term memory loss, continued to deny committing the offences.
However, judge Graham Buchanan sentencing MacKay, who is from Ross-shire, said: "You continue to maintain your innocence of these charges and were acquitted of more serious charges towards these girls at the young ages they were was quite disgraceful.
"A significant custodial sentence is the only appropriate form of disposal."
Judge Buchanan also placed MacKay on the sex offenders register.
The court was told that MacKay was likely to be freed from custody within a matter of weeks after serving the equivalent of 40 months in custody while awaiting trial.
Judge Buchanan was not able to give MacKay, who is considered a medium risk of re-offending, an extended sentence which would include monitoring in the community because the offences were committed before that legislation came into force.
Speaking after the case, Det Insp Andy Logan from the Public Protection Unit said: "MacKay exploited young girls over many years and today he has finally been held accountable for his actions.
"Whenever incidents of sexual abuse are brought to our attention they will be thoroughly and sensitively investigated by specialist officers, who will provide dedicated support, regardless of the passage of time.
"The bravery of MacKay's victims in coming forward has allowed for a robust investigation to be carried out and subsequently MacKay being brought to justice for his deplorable actions."
A pensioner who began sexually abusing two young girls more than 30 years ago was caught after one of victims began having flashbacks.