Dr Bex Lewis, Senior Lecturer at Manchester Metropolitan University and author of Raising Children in the Digital Age, was speaking after the media watchdog Ofcom found that children aged five to 15 spend an average of 15 hours a week online - more than they spending watching TV.
YouTube has become one of the most popular online destinations for children, with nearly three-quarters of five to 15-year-olds and more than a third of pre-schoolers regularly using the video site.
Ofcom also found children as young as three and four are spending more than eight hours a week online.
The regulator also found a third of 12 to 15-year-olds have seen hate speech online in the past year.
Speaking to Premier, Dr Bex Lewis said: "13 years old is the age that you can go on social media legally, so that's when you're more likely to be doing stuff unregulated, unsupported.
"Rather than thinking that a phone or a tablet are things you can just chuck your child on, the same as when you take them swimming you don't chuck them in the pool without armbands on.
"So online, you need to be sitting with your child, helping them to understand things."
And recalling a case study, she added: "One child who looked to be about six or seven, he said: 'I'm not allowed to press play on anything until Mummy's checked what I'm doing,' and I think that's sensible."
Listen to Premier's Aaron James speaking to Dr Bex Lewis:
