"De schildpadden slapen hier en de muizen slapen daar."
Translation:The turtles sleep here and the mice sleep there.
Very late reply, but others might be interested.
No, you cannot replace either 'hier' or 'daar' with 'er' in this sentence. Because a comparison is made ("here" vs "there), there is a natural emphasis on these words, hence 'er' (the unstressed form) cannot be used. :)