Things like this - at least from what we've experienced raising 5, seem to require the "right moment".
Hopefully that moment will come, probably in the form of a conversation that surfaces about someone 'dating' more than one person.
When those conversations appeared in our house we took the opportunity to present the fact that there are, and have been, many healthy, successful relationships that involved more than 2 people and it has a name. Then they're encouraged to go learn about it with an offer of some starting points if they want it. And expressed that if they run into something they don't quite understand - come ask ! Now, with the net, you can be sure they will come up with loads of resources in 5 min !
In our case, it comes with the warning that such relationships are more complex and more work, but for those up to the challenge the result can be worth it. But taking shortcuts won't cut it. Make sure you're up for it by learning first what's really involved.