easonm:
You say December 28 to "early January". How much time are you thinking of?
You ask which route would be best but follow up with the information that you are hoping to do the Primitivo. All things being equal, I would definitely echo the advice that the Frances has the best infrastructure to support first-time pilgrims, especially in the Winter. But if you are hoping to do the Primitivo, all things are not equal. What is drawing you to the Primitivo? As Peterexpatkiwi says, if you are just going to be walking the last sections of the Primitivo, you will be missing the mountains that many find to be the principal attraction of the route. But those same mountains are where you are most likely to find snow in the winter. On the other hand, if what is drawing you to the Primitivo is the possibility of seeing the Roman walls of Lugo, then that would be quite different.
You mention that you are going on "practice runs" with your daughter. I may be misunderstanding but I'll just say that running is not necessarily the best training for a long walk. I'd advise long walks of increasing length while wearing the loaded backpacks you intend to wear on your Camino.
In theory, the Invierno should be the best option for a winter Camino. The justification for the route is the idea that pilgrims took this route to avoid the winter snows over O Cebreiro. Hence it being called the "Winter Way". Unfortunately, it isn't in the same league as the Frances in terns of infrastructure, which is why people are not rushing to recommend it to you.