Timekeepers

"Okay Tom, you get to choose. What is your favourite period of history?" Thomas looked a bit bewildered at the bizarre twist the conversation was taking but answered without really having to think about it. "The dinosaurs," he said. The Professor let out a huge groan and Rebecca beamed with delight at the answer. "Dinosaurs? Oh very good Tom, excellent choice. Right, is everybody ready?" She stared ahead for a second as if trying to see something in the distance and then stepped forward, pulling the children with her. The three of them disappeared... Thomas and Emily Fletcher are quite normal children; normal in that their uncle mysteriously disappeared and now a medieval knight is trying to kill them. The only way to find out why somebody wants them dead would be to go back in time and ask their uncle, and the only people that can help are the teachers at the Tempus Viator School of Excellence. So it's very lucky that their new friends are time travellers...