To solve this problem, we can use the concept of divisibility and remainders.
Let's assume that there are "n" children standing in the circle.
Since the 5th child is directly opposite the 18th child, we can say that the distance between them is half of the total number of children minus 1.
So, the distance between the 5th and 18th child is (n/2 - 1).
Since they are evenly spaced, we can also say that the distance between the 5th and 18th child is (18 - 5) = 13.
Therefore, we have the equation: (n/2 - 1) = 13.
Now, let's solve for "n":
n/2 - 1 = 13
n/2 = 14
n = 28
Therefore, there are 28 children altogether.
The correct answer is B) 28.