This.
In answer to OP. I'd pick up the trash, make sure the neighbor noticed I did, and ask sternly to let the child go. If the neighbor resisted, I'd do all I could to get the neighbor away from the child and probably would call the police. I'd think an investigation would ensue and that not only my side of the story is the one that would be considered the absolute truth. I think I'd realize that by my getting physical with the neighbor, in trying to remove the neighbor from the child, I may have violated some laws, but would rationalize that had I not and had I waited for officers to take at least a minute to arrive on the scene, the child may have been harmed more.