Alanhuth - I've got a 4.5y/o, and a 1.5y/o, and as previously noted, I carry a Sebenza full-time. I've never had it just come loose, whether rolling on the ground, hanging over the side of my boat, etc. I'm sure it could, under some circumstances, as the clip is just a normal clip, but it's not been a problem for me. My normal approach with stuff like this is to keep an eye on it, and see if the kids end up with it, and then find a strategy to foil what actually happens. I find that trying to guess in advance what the little boogers will like, and how they'll go about getting their hands on it, isn't very productive.
My kids, especially #2, really like the knife, but it hasn't taken much effort to convince him that grabbing at things in dad's pockets isn't a good idea. He's also not quite tall enough to reach my pocket when standing. #1 is more than old enough to know that there are knives she's allowed to use with supervision, and ones that if she finds them lying around she should get a grown-up. She's pretty easy to work with, though - I don't anticipate it'll be so easy when #2 is 4y/o.
Also, I'd suggest taking a few moments to use the Sebenza on the cardboard box or something (drive it through a piece of 3/8 plywood?). It'll take off a bit of the ultra-sharp factory edge, and bring it into the range of 'very sharp knives', instead of the 'might lop off a whole finger sharp knife'. It'll also help convince you that you can really USE the knife. Shred the box into little pieces, whittle a new baseball bat, whatever... just take it out for a spin.