He initially denied a charge of sexual assault but the charge was later reduced to common assault and he pleaded guilty.
He also admitted failing to report to his nearest police station in August last year, which he is required to do annually because of his previous convictions.
The court heard that Cassie, of Croyland Green, Scraptoft, had three recent previous convictions for sexual offences.
In February 2014 he was convicted of sexually assaulting a woman at a bus stop.
In July of the same year he was jailed for 12 weeks for kissing a woman and stroking her leg in a pub, and in September 2015 her received a community order for sexually assaulting a woman after he sat down next to her on a bus.
The latest incident happened on July 8 of last year, which puts him in breach of that community order.
Judge Timothy Hurst told Cassie: “You’re 78 years of age and your behaviour is causing concern to a lot of people.
“It’s not a joke. I know you think these things are humorous.
“That kind of behaviour is shocking and frightening to her parents and it’s more than understandable that when the police found out about your record they were concerned about what your interests in that child were.
“It seems your sexual interest was in the mother – you made some comments to her.
“You sexually assaulted three different women in two years.
“I’m sentencing you for common assault and not sexual assault on the child and this offending attracts a community order.”
He gave Cassie a two-year community order with 30 days of rehabilitation activities.
He ordered him to pay £520 costs and sign the sex offenders register for the next five years.
The judge warned him: “The court is concerned about you.
“The court has sentenced you to prison before and your age won’t stop you being sent back.”