I hope the author does buy the whistle and scares the piss out of the kid. I bet it will work even better if the kid has some form of autism/sensory horse shit disorder.
But yeah, the author reeeeeally needs to find a way to keep the brat out of their yard. While I'm sure the breeders of said child are happy to let them trespass and destroy shit instead of watching them, I'm sure they will not hesitate to sue if their precious bastard hurts themselves on the author's property.
I'd be tempted to plant lots of flowers that attract bees. So when Junior goes to uproot them, he'll probably get a few stings. I also like the suggestion of planting thorny flora. Or COVER the plants in things like ghost pepper sauce, pepper spray, the hottest damn hot sauce you can find. Put it everywhere. Kids loooove putting their filthy fingers in their mouths and all over their faces. Let them get a little 5 billion scoville heat in their eyes, noses and mouths and they'll learn not to mess with the neighbor's shit.
Or in case the author is not fond of confrontation, get photographic evidence of the unattended child in the yard and call the police. Let the handlers get the kid back from the cops a few times, maybe it'll learn them to keep the little bastard corralled on their own damn property. Call every single time the child is in the yard and the police will do the confronting for the author.
Or put something in the Facebook Buy Nothing group about it.
"Free child! Approximately two years old, female (or whatever it is), appears healthy, no background check required! Pickup only at (author's address)." I bet you anything someone will come get it within like ten minutes. Let the neighbors deal with it when the kid goes missing.