In October 2016, Google made Project Fi much more accessible for a lot of people with the introduction of group plans. Today, those group plans are expanding even more with support for kids under 13-years-old.
Per Google's announcement post:
When you add a child to your plan, you'll both enjoy the Project Fi features you know and love—like easy data alerts, seamless switching between networks, and automatic connection to free Wi-Fi hotspots—in addition to the features already available in the Family Link app that you can use to set digital ground rules that work for your family.
As mentioned above, this feature is being added in conjunction with Google Family Link — a digital suite of tools that helps you manage and see how your kiddos are using their devices.
To add your kid(s) to Project Fi, create a Family Link account if you don't already have one, add them to your plan, and make sure they've got a phone that's compatible with the service.
Pricing is the same as other members in your plan, meaning you'll pay $15/month for your youngsters' talk & text and then $10 per each GB of data they use.