How do I get the Apple Music family trial?

I'm in the process of moving my music service from Spotify to Apple Music. I already have family sharing set up with Apple. I've added my son to the account. On the Apple Music page it says there is a 3 month trial for family subs, I clicked subscribe but it only gave me individual. I've tried to amend to a family plan but it's saying I'll need to pay the full sub amount from today. Can someone assist please so that I can get the full 3 month trial for family? Thanks!

[Re-Titled by Moderator]