There's a reason that Pelican cases have a lifetime guarantee aginst anything except "shark bite, bear attack and children under five."
Get them high and out of the way and hope they lose interest before they can climb.
I currently have a 3.5 year old and a 9 month old. I specially bought a centre speaker that I wasn't attached to, even though I glued the grill in, the eldest got to it within seconds and the dust cups have been cactus for years now.
I went for large towers, 65 pounds, and looked at any number of ways of fixing it to something but didn't get around to it, I figured the heavier the more difficult to push over. The grills have suffered as blackborads, but they haven't worked out how to take them off yet. The youngest most recently loves pulling out cables from the only one that has easy access. Even at 7 months he would crawl up to them, make himself upright, look around with a cheeky grin then start trying to get at them for a good chew. My family and friends are very amused that the kids treat my pride and joy so recklessly.
Have left all the AV gear exposed, the lighter stuff gets pushed a bit, but no great damage. I don't have the heart to rouse on them.