Google added family plans to Project Fi a long time ago, but it wasn't really built for the younger children in your family. The company is fixing that today -- you can now add kids under 13 to your Fi group plan through a Google account controlled using Family Link. You can track and set alerts for your little ones' data usage, and use Family Link to set time and app limits.
The timing is more than a little convenient given that Google just recently added the low-cost Moto G6 to its device roster. You now have both a relatively affordable device for the kids and a plan to help them use that service responsibly. You still won't have as many device choices as you would at major networks, but there 's now one less gap in Google's service options.