I don’t think the dragons were in the Queen’s chambers with Cersei and Tyrion. I might have to watch it again, maybe they were hiding in the back behind a bookshelf or something? Peeking in the window maybe? Disguised as The Mountain?
She could have killed Tyrion, and then took out the entire rebel alliance leadership in one go (seeing as she had them completely trapped in those pits), before Dany could even think dracarys.
Plus, she was aware of Jaime’s previous meet up with Tyrion, when there were no dragons, or unsullied, or Dothraki to be seen, so I suggest there are more reasons than just dragons for Cersei to not kill him.