Mobile data greyed out

Hi everyone, I'm trying to set up an Apple Watch Series 4 Cellular onto my phone to allow my child to use as part of the Family Sharing ability.

The watch pairs with my phone and sets up all correctly, however I cannot seem to go any further beyond the mobile data menu. On the next screen the "Set up mobile data" option is greyed out with a message reading:

"The Three account on this iPhone cannot be used to add a data plan to "Child's Apple Watch". For more information about Apple Watch data plans visit https://www.apple.com/watch/cellular"

I've been through all the diagnostic stuff with Three via their online chat, restarting both devices, unpairing the watch and pairing again etc., but have not managed to come to a resolution.

Three seem to have just given up claiming the watch is faulty? Surely there's got to be something else going on here?