The original sentence is: "To add challenge to the child’s looking for the fish, it can be half-hidden behind a stone."
Let's look at it step by step:
1. The phrase "the child’s looking" sounds a bit awkward because "looking" is used as a noun here, but it’s better to say "child’s search" or "child’s hunt".
2. The sentence can be simplified and made clearer.
Corrected sentence:
"To make the child's search for the fish more challenging, the fish can be half-hidden behind a stone."
This version uses "child's search for the fish" which is clearer and grammatically correct, and "to make... more challenging" which sounds natural.