"They were talking about rape. They were talking sexual activities that were pornographic," Sarah - not her real name - says, recalling some of the graphic messages sent to her child.
He had been playing Roblox online - where users build their own games and create characters with coloured blocks.
For Sarah, it initially seemed like an "innocent game".
She had turned on parental controls, so her son - not yet a teenager - could not send messages.
But, over time, she noticed a change in his behaviour.
He would no longer want to join in with family activities he usually enjoyed.
Concerned, she decided to check the game - and discovered he had been communicating with others on a third-party app.
It was at that point she realised her son had been groomed into sending sexually explicit images of himself.
Roblox told the programme it was unable to comment on individual cases but was committed to protecting the online safety of children.
It added: "It's extremely important to be aware of these chat apps, particularly [those with an] 'overlay' feature making it appear to be part of whatever game is being played."