Honey contains spores of Clostridium botulinum, not live bacteria. And as such, honey cannot in itself cause botulism.

But the intestinal microbiot of infants (not their immune system) is not necessarily developed enough, and as such Clostridium botulinum can colonize their intestine (it strives in anaerobic environment) and then they can develop a special kind of botulism where the toxin is actually produced in their own body (as opposed to ingested, like in regular botulism).

It's not the only way a child can stumble upon the bacteria's spore though.