Suppose you have 5 slices of cakes.
If you share it equally among 5 people, each person will get 1 cake.
Now, suppose you have 0 slices of cakes.
If you share it equally among 5 people, each person will get 0 cake.
Why is this so? This is simply because there is no cake to share in the first place! Thus, everyone received nothing, i.e., zero.
The difficult part is explaining zero divided by zero, so avoid that first until your child understand the concept of why zero divided by any non-zero number is still zero.