Simple some things are done no matter what you teach them, you end up doing it and you have to learn from experience. This is one of them in my opinion, it is dang hard to hold their hand forever and watch them like a hawk to make sure they don't move the blade the wrong direction and rely on the lock because it is such a small subtle move a lot of times. With a slipjoint you get an instant feedback of the knife wanting to collapse back into the handle which is may or may not do depending on the force. That is also where they will most likely do their screw ups will be and have a blade collapse or almost collapse on a light duty task where the force will be just the weight of the blade snapping shut or slightly more if you teach them right and not to do anything stupid like try to stab pieces of wood, etc. The other time they will most likely cut themselves is closing the knife by having their fingers in the way as it snaps shut, once again it is not something you can always prevent as it happens fast and will usually result in a band aid and nothing more. (If you don't fully understand this I urge you to grab a slipjoint and put it in a piece of thick cardboard halfway and push upwards SLOWLY till it starts to disengage, you will see it and feel it. Feel free to wear leather work gloves. That is the feedback I want them to learn from as it's hard to always spot and only takes a couple pounds of pressure to do that at most. Now do that to a locking knife and it is rock solid and doesn't tell you that your doing something wrong.)
On top of the feedback they get with using the knife in such a way that they rely on the lock and keeping their fingers clear while closing the blade it will be harder to use it as a pry bar. That is due to the nature of the knife, using it as a pry bar is quite a bit harder because if you wiggle it slightly the blade will want to collapse into the handle. These are the skills you want to develop and enforce. A locking knife will allow you to have a lot more leeway with using a knife improperly by letting you do these things more and let them develop bad habits. That has far more potential for serious injury when something does fail than getting a few band aids when your 8 because you used your slipjoint improperly.
And if your telling me you can keep track of a child 100% of the time so they NEVER get hurt I say your full of it, kids get hurt all the time, scrap knees, bumps, bruises, etc. It can take less time for them to screw up and get hurt than for you to yell "NO" for something to happen. You should be teaching them what to do and not to do, but also not be afraid to let them learn from experience when you can stack all the odds in their favor for them not getting seriously hurt if it means they be better down the line. It's kind of like riding a bike, it may be safer to keep them with training wheels but one day you have to take them off and when you do you should make their they are decked out in padding and have a few band aids ready as they will fall a few times before they learn how to stay up no matter how much you instruct or supervise them.
At least you aren't just the "1 and done" type.
