My children cannot download any apps to their watches. We cannot figure out why.

They already have their own cellular plans and phone numbers on the watches. Calling is not a problem. Downloading apps is the problem. I brought one to the Apple Store yesterday and the employee spent a very long time with me, was super helpful, asked others three different times and the conclusion is they were stumped why it wasn’t working as well. The only suggestion they had at that point was to unpair and repair the devices and if that didn’t work - call apple support.