Well, obviously the only real issue is Jonah's safety - but you could try spinning the conversation to something like,

"my toddler and I will be in the back yard and I'm trying to figure out a way to keep him from spooking your dog - what would you suggest?" - this way, they get to believe that you care about their welfare (even though you don't , and who knows, they might have a good idea.

If they're not cooperative - put up the fence.