I'm curious because I've always assumed that Cersei would burn the city to the ground AFTER losing even her last child, but Tommen is alive (So far) and so whatever happens?

This show LOVES PAIN. If there's a prophecy stating that Cersei will watch all her children die, I fully expect them to come up with a way for her to (accidentally or not) kill Tommen. Say, she thinks she's made sure that he won't be present for the Big Sparrow Bonfire, and he comes up with a cunning plan and sneaks in the backdoor or something...