Rapist Leslie Foster, 63,exposed himself to a seven-year-old from Heysham, who had gone to the Toys R Us store on Deepdale Retail Park in Preston to spend her birthday money.
The schoolgirl was temporarily separated from her dad as she looked at the doll aisle on February 1 this year.
Foster, who is on a lifetime licence after committing sex offences in the 1960s and 1970s, had only been released in 2012 from a life sentence for rape.
Police said he had been in and out of the store for five-and-a-half hours after catching a bus to the retail park.
He had changed his appearance with a bobble hat and glasses before he approached the girl and exposed himself.
He asked the girl to show her knickers and said he would buy her any toy she wanted.
The terrified youngster ran away from him and raised the alarm with store staff who tannoyed her dad and brother.
The girl’s dad, who cannot be named for legal reasons, said: “I wanted to kill him.
“My daughter wanted to buy a kind of doll Toys R Us do with her birthday money.
“It happened in 30 seconds. My children wanted to look at different aisles.
“My daughter was in tears and the staff told me what happened.
“I was relieved it hadn’t been any worse. It doesn’t seem to have affected her too much luckily, but it may do when she’s older. It’s really affected us.
“The staff in the store were brilliant.”
Foster, of Spendmore Lane, Coppull, Chorley, admitted exposing himself and inciting a girl to engage in sexual activity. He will be sentenced at Preston Crown Court on Friday.