Currently your supported option within Zwift is to buy a monthly membership for the winter months since there is no longer a U16 free account, you can cancel when the team moves outside and restart it next winter if you don’t want to do an annual sub (they will keep their badges/achievements etc. when they start paying again the next year). It’s unfortunate that those child accounts were removed, and there is a large request thread for a family plan that would reduce the cost of two or more subscriptions, but that is not currently available.
I wonder if the mountain bike team can possibly sponsor the membership in some way? The U16 accounts going away is going to be a bigger problem as more members age out and new members join, so they will either expect folks to buy a subscription as part of the team, or move to a different platform… Maybe do a team bake sale to support Zwift memberships for the team 
Anyhow, it’s unfortunate the U16 accounts no longer exist, but my understanding is it was due to the complexity of supporting the regulatory requirements for child accounts in various markets etc.
Edit: Here is the family plan thread I believe: Family and Child Plan Request - Feature Requests - Zwift Forums
Edit 2: And here is a statement from Zwift on the U16 accounts not in plan: Re-instate U16 Accounts on Zwift - #5