Lita Dawson and boyfriend Nicholas Harrison, both 51, were spotted on top of each other on Seaford Beach, in East Sussex.
One five-year-old boy told his shocked mother: 'Mummy, that lady was doing push-ups on that man.'
Eastbourne magistrates heard the pair had downed half a bottle of vodka before pulling off their clothes.
Prosecutor Heather Salvage said: 'They were in full view of everyone and didn't seem to care.
'One lady was at the beach with her son, who was only five, when she became aware they were having sexual intercourse. She tried to shield her son's eyes.
The unemployed pair were arrested on the spot when outraged parents reported them to police.'She said it made her very angry.'
Magdalena Biglou, defending, said Dawson initiated the encounter on April 29, although both were to blame.
She said: 'There's not much that can be said really. It's quite apparent that their behaviour was was unpleasant, inappropriate and quite disgusting.
'The main instigator was the female. It was the female who was on top of the male.
'He asked Miss Dawson to stop a number of times but essence his sexual arousal took over.
'He is totally disgusted in himself and his behavior.'
The court heard the couple have since split up, after Lita Dawson punched him in the face.
They both admitted outraging public decency and were each jailed for three months.