Susie is the child — specifically, the only child — of her mother.

Why this riddle works

The trick is in how you read “no children.” In many versions, people play loosely with language so that:

  • “No children” is taken as “no children except Susie,” implying Susie is the only child.
  • The question “Who is Susie?” is then answered: she is her mother’s sole daughter.

An example way to phrase it more clearly would be:

“If Susie’s mother has no children other than Susie, who is Susie?”
Then the answer is obviously: her mother’s only child.

Why people argue about it

On forums, you’ll see people joking with other answers like “Susie is an adult” (so her mother has no children anymore), or other playful twists, which is why this keeps circulating as a little logic/wordplay puzzle.

TL;DR: In the most common intended reading of the riddle, Susie is her mother’s only child.