It is of course possible I missed a story but before January 3rd there was little to no mainstream news coverage. It is quite frustrating to find accurate articles now as many sites are falsely returned in date range Google searches. All "mystery flu-like pneumonia" stories I've found reported in December 2019 are medical news so it is extremely unlikely someone would come across such an article as part of normal news reading.
Especially for my 8 year old son. He is old enough to understand things are not normal but young enough to not fully understand the gravity of the situation or have memories of much outside of his little bubble.
Apart from that I remember the first reports from China in December 2019.
https://bfpg.co.uk/2020/04/covid-19-timeline/
I too remember talking about it on Christmas.