Set up cellular on a standalone Apple Watch for a child on a different carrier than iPhone

My iPhone is on AT&T prepaid, which does not offer standalone apple watch plans. I'm setting up an apple watch for my child using the family member setup here: Set up Apple Watch for a family member - Apple Support. I had already paid for a standalone plan on US Mobile and registered the IMEI. On step 7 of those instructions, "Set up Cellular", I could not activate cellular for the watch on US Mobile - the watch would not obtain an eSim. Every time attempted it, it would ask me to log into the AT&T Prepaid portal. I had to work with US Mobile customer support to have a temporary US Mobile Warp eSim assigned to my iPhone, then disable my AT&T prepaid eSim to get through the child watch cellular setup.

Is this how it's supposed to work? Is there an easier way to do this? I need to set up another watch and would like to know the best process, because without a savvy customer service agent at US Mobile, I would have been stuck.