Coral Jones and Mrs McCann, mother of missing Madeleine, unveiled a digital billboard at King's Cross station in London promoting an enhanced Child Rescue Alerts system to inform the public about missing children whose lives are considered at risk.
The scheme will use social media including text messages, email and digital billboards across the UK in addition to traditional broadcast media to issue the alerts when it is launched on Sunday, International Missing Children's Day.
The scheme, the responsibility of the National Crime Agency (NCA) in partnership with the charity Missing People and the company Groupcall Limited, already has 25,000 people signed up.
Organisers say the scheme will be able to issue localised and national alerts and has a target of a million to sign up by the end of its first year.
Chris Hedges, manager for Missing and Abducted Children at Ceop, part of the NCA, said: "It enables us to disseminate messages either at a very localised level or nationally quickly and effectively whenever there is a particularly highly endangered child we are trying to locate.
"We are encouraging the public to sign up to the scheme so they can receive alerts, hopefully on rare occasions when we use the scheme."
Mrs Jones, 42, urged people to sign up in an attempt to help other families of missing children.
"The search for my beautiful April ended in tragedy and that is why I want to do everything I can to make sure that the next time a child is taken or in danger we can bring them home safely," she said.
Mrs McCann also urged people to sign up to the alerts.
"When a child is abducted, families are devastated and entire communities are torn apart. The agony of not knowing where your child is, is almost impossible to imagine. The helplessness is at times overwhelming. But there is now something we can all do to help."
Mark Bridger, 47, was found guilty of five-year-old April's abduction and murder last year. He snatched the little girl from outside her home on an estate in Machynlleth, Mid Wales, while she was out playing in October 2012.
The paedophile former lifeguard was given a full life sentence when he was jailed at Mold Crown Court in North Wales last year.
Madeleine McCann, then nearly four years old, disappeared from her family's holiday apartment in Praia da Luz in Portugal's Algarve on May 3, 2007 as her parents dined at a tapas restaurant with friends nearby.