I recently asked Zwift if they do a family discount. My husband and daughter would love to have their own account but there is no way we could afford £39 a month. They suggested posting the topic on the forum. The more people requesting it, the more likely it would be put into fruition!
There is already a few Feature Request for this: https://forums.zwift.com/search?q=Family%20Discount
This is the main one: Family and Child Plan Request
Make sure you vote up the existing requests for this feature.
I am going to lock this thread since there are already more than a few requests for this.