The woman and her child, aged just four, were on their way back home after visiting friends on the south coast on Sunday when she pulled into the lay-by, near Hannington, at about 2pm to take her dog for a walk.
The 44-year-old says she was left distressed by what she saw.
She said: “We witnessed the horrifying spectacle of several elderly men wandering around naked.
“My daughter was very upset by this.
“We then saw two men having sex in a coppice, and later on, more sexual activity.
“At no point did any of these men attempt to cover up.”
She said another man was performing a sex act in full view of passing traffic.
He said: “My daughter is traumatised by what she witnessed.”
The woman and her husband reported the matter to the police.
In an emailed response, Sgt Mike Grady, of Daventry Rural Safer Community Team, said: “We have long been aware that these lay-bys have been used as a gay meeting place, however, we were not aware that the area was being used beyond initial encounter.
“I will brief my staff to patrol the area and deal with any offences they find.”
The woman’s husband, who has recently moved to Kettering with his family from Wiltshire, said: “As soon as my wife got back to the car she called me on her mobile.
“She told me my daughter was in tears.
“Someone had even left an obscene message on the car windscreen,” he said.
“The whole incident was shocking.”