Trying to set up Apple Watch For your Kids

I've purchased an Apple Watch SE GPS/Cell. I am trying to set up Apple Watch for your Kids and hitting a brick wall. Neither Apple or a Telstra store were much help (face to face) in providing step by step instructions so after two visits to Apple and one visit to Telstra I checked the Apple website and purchased the watch. Today I have gone online and purchased a yearly plan with Better Roaming, as per what both Apple and Better Roaming's websites stated, to get the watch up and running. Family Sharing was set up previously and when I didn't receive a notification on my phone after purchasing the Better Roaming plan, I manually did it and then got the message during setup on my iPhone that my iPhone service provider does not support the plan I had purchased from Better Roaming. What do I do and why can't anyone tell me straight exactly how I get this up and running? I have logged a ticket with Better Roaming and they have come back to me (quite quickly) but with trouble shooting tips telling me to do the same as i have done initially (except to click on the Better Roaming hyperlink on the Apple website and 'continue set up', which only takes me to my active account on Better Roaming's website. Waiting to hear back from them (again).

What type of service connection and with which exact service provider in Australia do I need for this Apple Watch for Kids setup to work? Please help!