Why do you think Bruno’s father enquired about what Bruno knew about the other side of the fence?

In my opinion, Bruno's father inquired about what Bruno knew about the other side of the fence in order to see exactly how much he was aware of. When he realizes that Bruno knows there are children on the other side of the fence, he becomes aware that Bruno knows more than he intended. This helps him to make the decision that Bruno and Gretel should leave Auschwitz and return to Berlin with their mother.