I've been through four kids and now half a dozen grand kids. The "fence" works to an extent, but as they grow it becomes less effective.
If you have full size speakers with threaded inserts for the spikes, you have threaded inserts that will work pretty well to hold a broader base to the bottom of the speaker. It's just a matter of choosing a base material and popping the holes.
I'm now saying that this will withstand anything that you could throw at it, but it will help to prevent those blink of an eye "oops" situations.
I remember having to deal with this with my last child. I had Studio 40s on stands and those weighed 40lbs each. They were also pretty easy to knock over.
Personally, I just gave up on the whole situation. My wife wanted an entertainment center to keep things up and out of the way as well. I just resorted to using really small speakers on shelves in the entertainment center. I'm running Silverline Minuets now.