The error in the sentence is B. Dangling modifier
The introductory phrase "Without knowing where to go" is a dangling modifier because it doesn't clearly and logically modify the subject of the main clause ("the two children"). It suggests that the children were without knowing where to go, which is not the intended meaning. To correct the sentence, you could rephrase it to make the modifier's reference clear, such as:
"While not knowing where to go, the two children wandered through the forest, got lost, and a witch found them."
This revision clarifies that it was the children who did not know where to go.