Brad Summer, from the US, explained his daughter had an account on Musical.ly, a social network app for video creation, messaging, and live broadcasting.
His daughter was sent messages from an app user claiming to be nine years old, asking to her to send pictures of herself without a T-shirt on.
“Please, tell your kids to let you know if anyone ever asks something like this, let them know it’s okay to tell you,” Summer wrote on Facebook on 12 August.
“It has helped us in this situation.”
Summers shared screenshots of the messages between his daughter and the unknown app user. He said his daughter only used the app on his and his wife’s phones when they were around.
“Make some pics without T-shirt,” one of the messages read. “I like to see your body without T-shirt.”
His daughter replied saying she couldn’t do that and received the response: “It’s secret between us only”.
Summers encouraged people to share the screengrabbed exchange to help others.
“I know many will blame us parents for this happening,” he wrote. “But we never thought like predators and I guess we were naive in thinking that our daughter was safe on what we thought to be a kid-friendly app.
“We have learned the hard way. I ask that you not judge us (many still will) but let our experience teach us all.”
Since first sharing the Facebook status, Summers updated it to say a police detective had found the IP address of the person who sent the messages to his daughter and they were continuing to “make progress”.
He also thanked people for sharing and spreading the warning to other parents.
“By sharing, we have reached and informed so many families to be on the look out for these people,” he wrote. “This story has spread through almost every continent and the replies have been overwhelmingly touching.
“From those who comment, to those of you who send me a PM, I read each and every one. I have been given advice, other sites to look out for and most appreciated, the encouragement to keep fighting.”
His post had nearly 85,000 shares within two weeks of being posted.
One parent commented: “Many parents look into these sites but we don’t fully see what may be done or don’t expect what could happen.
“I hope when the investigation comes to a close this person will find themselves with a very hefty sentence.”
On the terms and conditions on Musical.ly’s website, from July 2016, it states: “The service is not for persons under the age of 13.”
However Internet Matters, an organisation guiding people through issues children can experience online, stated there has been confusion over the minimum age.
They advised: “If your child is receiving unwanted messages or comments, they can block the user on the app. You can block a user by going to their profile and selecting the icon with three dots on the right hand corner and then choosing ‘block this user’.
“If your child comes across any content that is not appropriate they can press the icon with three dots on the side of the screen and select ‘report abuse’ from the list of options.”
Webwise, an Irish online safety organisation, also stated on their website: “There seems to be a little confusion over the minimum age users should be.”
They recommended to go into privacy settings and switch off the location information. App users are also able to make their account private and request only their friends can direct message them. This is advised.
HuffPost UK contacted Musical.ly for comment and will update this piece on their response.