This past Saturday I took my son and daughter out for a day sail (13 and 12 years old respectively). The waves were all of 3-4 foot with a few higher swells here and there. Wind was in good supply at 15-20 knots. Since my daughter was already wigging a little bit about the wave action, I decided to only unfurl the jib, at least initially, so as to limit the amount of heel the boat took on.Kids being kids, they fight over any and all things, including who is going to do boat chores. They fought over who was going to go below and get the first round of beverages. They fought over who was going to stow the fenders after cast off. They fought over which one was going to go below and pop a CD into the sound system when I asked for somebody to put some music on. My daughter eventually gave in and grudgingly left her seat to do this last chore.While she was down below the boat hit a couple of larger swells and bounced sidways a few times. With no warning, the boom crashed down on the side of the companionway. The end of the boom hit right where my daughters head would have been if she were sitting in the cockpit (the kids prefer to sit lounge style with their backs to the bow, taking up a whole bench seat). The boom didn’t just drop. I came crashing down, accelerated by the action of a wave. The topping lift had let loose and with no main sail up to stop it, down came the boom.This just completely unnerved me, as it did the kids. My daughter was sitting below sobbing. I furled up the jib, secured the loose boom, fired up the diesel and headed back to port. I was shaking and felt sick. A few seconds difference in time and I would have had the grotesque job of trying to contain my daughters brains in her crushed skull while the my son called for help that probably would have arrived too late. Then having to later explain all this to my wife… I don’t even want to think about it! I am sure God was looking out for us on that day. Thank you Lord.When I finally got back to port I looked at the failed topping lift. My first thought was that it had come unhooked from its cleat (I’ll explain below). Inspection proved this was not the case. The line connected to the boom had actually frayed and broken. My boat is a Hunter 31 with an Isomat rig, and as far as I know, the topping lift is the original design. It consists of a stainless wire hanging down from the mast head. At the end of the wire is a small block. The block is about 3 feet above the normal height of the boom. There is a 1/4“ line that connects to the boom via a stainless eye riveted to the port side, goes up and around the block on the wire line from the mast head, down to a small cheek block on the starboard side of the boom, and then along the length of the boom to a small 3” cleat to which the line is tied off. The eye splice in the 1/4" line on the port side let loose. My point here is three fold. First of all, if you have this type of topping lift setup, please check its condition. In my case the line did not give any indication it was about to fail. Though it certainly was not new looking, it did not appear frayed or worn. It’s failure was a complete surprise to me. The consequences would have been devastating had my daughter not moved from where she was. Secondly, since this has happened I’ve been trying to think of how it might have been prevented. It appears that either the eye splice failed or the line wore through where the splice connects to the stainless eye strap at the boom. The best idea I have come up with is to add a level of redundancy by attaching the main halyard to the boom end whenever the main sail is not in use. Short of going to a solid vang type of device, how else might this be prevented in the future?Lastly, I now need to repair the topping lift system. I never did like the way it was designed to begin with because the cleat that secures the line is about half way down the length of the boom. It makes lifting the boom up high enough to clear heads a two man job, one to lift the end, another to stand on the foredeck and cleat it. Any suggestions for a different design? Moving the cleat back so it is accessible from the cockpit is a start. What about a different type of cleat? What about improving the eye splice to metal eye strap connection with an intermediate shackle of some type? Any other ideas anyone?