That's a late answer, still I hope it will help you.
Sweets are not a metaphor, since it can be read in the manual that childrens considered Gregory as 'the man who gave them sweets'. Though, I don't think he used those sweets to kidnap childrens. In my opinion he began to come to the orphanage to see Jennifer escaped from his house and he brought sweets for her and the other orphans (though he was not authorized to cross the gate).
It's hard to say however whi we can find a warning that says "Stray dog gives us sweets".I think Wendy wrote it in order to make the orpphans fear him and avoid him (telling them he was stray dog), in order to get closer to him more easily and later start training him.
The legend of stray dog is quite obscure indeed, but I don't think the details are not a big deal to understand the story, so I invite you to make your own interpretation.
Yours