Is it normal for a child to do this while on the toilet?

When I was a very small boy I had a very vivid imagination and I would often have flights of fantasy. When I used the toilet I used to think there were 2 spiders that lived down there, hidden from sight that would be scheming ways to make it up to the top and bite my ass. It makes no sense looking back on it now, cos how would they have survived submerged in water?
Nevertheless, these spider bandits (I actually imagined them with criminal personalities) plagued me every time I used the toilet, to the point where I would use the toilet by elevating myself above the toilet seat and looking through my legs for surveillance. I always imagined them down there, saying "damn we've been foiled again, he's onto us!"
Looking back, I was a fearful little kid. I used to run and launch myself into bed because I was scared the monster underneath my bed would grab my feet and pull me under if I walked up to the edge to get in.
So to answer your question, normal. Childhood imagination is a weird and wonderful thing.