My dad is very fond of this one story and he told it to me as he remembers it. Trouble is he doesn't remember the name or the author as he read it probably back in the '70's. Here's a synopsis.

A scientist goes back in time, maybe Jurassic period, maybe even earlier.
While there he encounters a boy and a girl whom he rescues from some animal threat. During the night they talk and he learns that they are not of this earth. Also, the girl is a genius mathematician and the boy is a genius mechanic. The next day he helps them escape and afterwards finds himself wedged in between two dinosaurs. Just as he is about to say his prayers and hope for a quick death he realizes that one of the dinosaurs is mechanical and it is guided by the people from his laboratory. Once inside he also recognizes that the chief pilot is the older version of the girl he just rescued, while the dinosaur is obviously built by her brother....

Does anyone have a clue as to who wrote this? I initially thought maybe Paul Anderson due to his Time Police series but that was a dead end.