Parents should start to talk to their children about online pornography and sexual harassment when they are as young as nine or 10, ideally before they have a smartphone or social media account, the children’s commissioner for England has said.

Dame Rachel de Souza urged parents to “talk early, and talk often” to their children in an age-appropriate way, so they are better prepared for the risks of the online world and will find it easier to raise difficult issues further down the line.

Concern has been growing about the ease of access to and damaging impact of online pornography on young children who often browse the internet unsupervised and are exposed to material that shows degradation and violence towards women.

The issue was recently brought into sharp focus by the Everyone’s Invited movement, which revealed widespread sexual harassment and abuse in schools, fuelling concern that online porn can influence real-life behaviours and attitudes around gender roles, sex and consent.

Rather than waiting until children are teenagers, De Souza wants parents to open the conversation much earlier and has produced a guide to help them negotiate tricky conversations about children’s online behaviour, using advice from teenagers and young adults based on what they wish their parents had known.

“The overriding message you’ll see from our focus group is talk early, talk often,” she said. “You might be surprised how early our young people felt parents need to start the conversation. But kids want an age-appropriate conversation that evolves over time in line with their growing maturity.

“My advice to parents and carers is to create the culture before the crisis. Children have told us they want their mums and dads to create a safe, judgment-free space for them to talk about these issues. It’s better to do that before you hit a problem rather than trying to create that mood while you’re dealing with one.”

This week the Grammy-winning singer Billie Eilish revealed the damaging impact watching pornography from an early age had on her. She started at 11 and developed an addiction that gave her nightmares and made dating difficult.

“I think porn is a disgrace. I used to watch a lot of porn, to be honest. I started watching porn when I was, like, 11. I think it really destroyed my brain and I feel incredibly devastated that I was exposed to so much porn,” she told the Howard Stern Show in the US on Monday.

De Souza’s guide highlights the gap between what parents think and their children’s reality. Only a quarter of parents think their child has seen pornography online, but in reality more than half of 11- to 13-year-olds have already seen porn, many came across it by accident and 62% say their viewing of porn is mostly unintentional.

“Talking to our children about this issue can be hard,” said the children’s commissioner. “Parents tell me they sometimes feel uncomfortable, not just because of the sexualised nature of the topic, but also because their children know more about technology than they do.

“For mums, dads and carers who grew up without smartphones, this whole world can feel bewildering. But children want to talk to their parents and carers about this. We know this because they’ve told us. And that’s what is at the heart of this guidance.”

As well as online pornography, the guide – which is called The Things I Wish My Parents Had Known: A Guide From Teens and Young Adults on Dealing With Sexual Harassment Online – also looks at the pressure on young people to send nude pictures and sexualised bullying.