When I have a walker(s) that I don't think will walk nice or I'm afraid they're a runner, I use the backpack harnesses. It buckles around their stomach and I attach the other end to the stroller or to my wrist. It has 2 buckles so if they were to undo it, I'd notice before they got the 2nd one undone. They have always worked wonders for me.
The only thing that I don't like about them is that it does create a lot of "fake" independence and I sometimes wonder if it will create a habit of the child not wanting to hold hands with an adult when needed. To try and stop this, I make the child hold onto to the stroller as much as possible (some sidewalks don't always allow room for this) and especially when we cross streets. To shorten the "leash", I just wrap it around a few times on the stroller handle or my wrist.
They sell them at Walmart for $10 and come in cute little animal options.