A Snapchat child sex groomer being hunted by police sent photos of his genitals to a 14-year-old girl.
The person with username CraigDavis1999 also messaged: "I've seen your pictures and I'd like to see what's under your clothes", the mother of the girl said.
It comes after Thames Valley Police warned parents in Berkshire of two online profiles targeting young pupils.
Children's charity NSPCC said 183 child sexual offences were committed online in the Thames Valley area last year.
The mother, who cannot be named, said her local council had sent messages to schools in Bracknell from police, asking to check pupils' phones for two Snapchat profiles that had approached children - CraigDavis1999 and James Hammond.
Her daughter then told her the former sent her indecent messages and photos via Facebook.
Her mother said the experience had left both "shocked and scared".
"You can't trust anyone," she said, "when my daughter goes out anywhere I'm worried he may be following.
"There are no pictures of the man himself so you can't look out for him.
"You just don't think it's going to happen to your child and you just don't know how to deal with the situation."
An NSPCC spokesman said it was "currently far too easy for adults to contact children online" and is calling for tighter controls on social media.
The charity "wants to see a clear set of minimum standards that all social media companies will be held to account to, including ... bespoke accounts for under 18s."
Katharine Hill, UK director of Care For The Family charity, said it was important for parents to talk to children about the dangers of online grooming.
"Parents need to put ... safeguards on devices to protect them. However those simply aren't enough. They will soon be on public wi-fi or a friend's device without protection," she said.
No arrests have been made so far.