To add onto the idea of the Zwift kids’ accounts (which it sounds like you’re aware of), I use the Meetup functionality in there, along with the “Keep everyone together” option, to let my kids ride with me in Zwift. They’re a little younger than yours – 6 and 10 – and just can’t really keep up with me. With the meetup and KET, they’re able to ride along, even up hills, through the volcano, etc., which definitely adds to the fun factor for them.
This has, of course, led to the need to somehow get 3 bikes into my office, along with having 3 devices to run Zwift. 