Deb Schmill has become a fixture on Capitol Hill. Last week alone, she visited the offices of 13 lawmakers, one of more than a dozen trips she has made from her home near Boston over the past two years.

In each meeting, Ms. Schmill talks about her daughter Becca, who died in 2020 at age 18. Ms. Schmill said Becca had died after taking fentanyl-laced drugs bought on Facebook. Before that, she said, her daughter was raped by a boy she had met online, then was cyberbullied on Snapchat.

“I have to do what I can to help pass legislation to protect other children and to prevent what happened to Becca from happening to them,” Ms. Schmill, 60, said. “It’s my coping mechanism.”

Ms. Schmill is among dozens of parents who are lobbying for the Kids Online Safety Act, or KOSA, a bill that would require social media, gaming and messaging apps to limit features that could heighten depression or bullying or lead to sexual exploitation. The bill, which has the greatest momentum of any broad tech industry legislation in years, would also require the tech services to turn on the highest privacy and safety settings by default for users under 17 and let youths opt out of some features that can lead to compulsive use.

Modeling themselves in part on Mothers Against Drunk Driving, which pushed for the 1984 federal law mandating a minimum drinking age of 21, about 20 of the parents have formed a group called ParentsSOS. Like members of MADD, the parents carry photos of their children who they say lost their lives because of social media, and explain their personal tragedies to legislators.