My three and a half year son has just watched his second Dr.Who episode.
I felt fairly safe letting him watch these two without previewing them first as although the Sontarans are certainly alien and rather ugly, they are not the most frightening of Dr. Who bad guys!
He enjoyed the show and now wants a Dr.Who action figure! Yay, another convert!!!!
I will probably record and preview future shows as I do not want to put him off and he is a little young for watching from behind the sofa!