You'd be surprised at how small dogs can jump a fence. My old Pomeranian who passed away now, used to jump the 6 foot fence. She would get a running start and basically just run right up the side and then scramble her butt over the top. Took us a few to figure it out, she'd go over the fence around the front of the house and then bark to come in through the front door...she was a demon. However, most dogs as small as her probably would not kill a chicken, or at least not a bunch of them.
Coyotes I'm sure would have no problem jumping it.
I'm very sorry for your loss. We keep our pitbull and our Aussie out with the chickens from the time they come out in the morning till it's time for the coop to get closed up for the night. Keeps unwanted guests out of our yard, and we don't forget to close up the coop because they want in at dark fall and will bark at our door when it's time. The Aussie is 19 now and probably wouldn't care if a coyote walked across his back and then snacked on chickens next to him, but our 12 year pit still keeps guard well. (wish they didn't get old, I really don't relish finding such good dogs again)