It was called "Cycle Pods", which would be mini-critical mass rides for commuters. You'd probably need between five and ten riders who cycle the same route each day at a roughly similar pace, so you could register with a route and see whose rides were similar to yours.
The other idea was even more loose, where you'd wear something like a tabard with an identifiable "cycle pods" logo where anyone who was wearing one was effectively advertising that they were open to riding with other people. I preferred this version because it's simpler and decentralised. Anyone could make something with the logo on it, so no purchase or registration needed.
I love the idea of doing it with kids, but honestly some of the reactions we got from car and van drivers with the Critical Mass rides, even when we were going at a decent pace, were incredibly dangerous, and I'd worry about that with kids unless the police were involved, as they were in this Bicibús. We had to have outriders looking out for car drivers who'd try to drive into the middle of the mass. There's something about cyclists taking over the road that really provokes some drivers.