She spent months embedded with tweens and teenagers, sitting next to them as they lived their digital lives. They exchanged messages with friends, fell in love with strangers, were bullied, watched pornography and played video games. The documentary features the children describing how they felt simultaneously interested, hooked, influenced and repulsed by online services.

“I was worried about some of the feelings they were having,” Lady Kidron said.

By 2014, Lady Kidron had started a foundation, 5Rights, to promote children’s digital rights. But she felt frustrated with major tech companies. They scrambled to handle one problem after another for children on their sites, she said, but appeared unwilling to make major changes to try to avert such problems in the first place. She concluded that the only way to give children more privacy, more freedom and more control over their online experiences was through regulation.

In 2017, she proposed the idea of children’s online protections in Parliament as an amendment to a national data protection bill. The bill generally called for protections for children’s data, but Lady Kidron pushed to detail those protections.

In real life, “we’ve already decided that children have rights,” Lady Kidron said. “For me, it’s all a part of just making tech normal.”

The proposed rules are called the Age Appropriate Design Code. Among other things, they would require that online services turn on by default the highest privacy settings for all minors, preventing things like automated location tracking. They would also require the services to automatically turn off techniques that could push children to stay online longer or provide the companies with more personal data than necessary.

The changes present a challenge to platforms like YouTube and Instagram, which have said their services are not intended for people under 13 — even though tens of millions of children use them. The British code would apply to all online services, including social networks and messaging apps, that are likely to have users who are under 18.

“It is provocative to technology companies,” Ms. Denham, the information commissioner, acknowledged. “It is going to change systematically, and on a systems level, how services are delivered for kids.”