He is described as white, of lean build, wearing a woolly hat, jeans and jacket.
The family were left shaken and very upset by the incident.
PC Connor Crespin said: “We’d like to speak to anyone who was in the area yesterday and may have heard or seen this incident or the male on the bicycle.
“We’d especially like to speak to two people who were nearby at the time and who we believe witnessed the whole event. If that is you – please come forward.”