You got plenty of good tips already.
I am an Opinel user myself (everyday, not only when hiking); I use it to cut food, open envelopes, fish insects out of my drink, replace dull pizza knifes in a restaurant and whatever else you can think about. What I did with my kids was this:
When they were very young (like 5-7 or so) they were allowed to handle the knife, but I made it an act of importance. I.e., I would not just throw it over to them and let them do their thing. I would hand it to them slowly, while looking at them steadily and talking earnestly. I very much made it an "adult" thing for them.
They would very consciously be allowed to study the knife, open the lock, unflip the blade. I would encourage them to touch all parts of the blade (by showing more than telling, of course). The small pain when putting the tip to skin (without drawing blood, obviously) and the resistance when moving a fingernail across (not along, obviously) the edge. Folding the blade back in.
From there on, they were allowed to do stuff with the knife; maybe cut an apple or some meat etc. on a "need to do" base (i.e., not playfully). As my knifes are very sharp indeed, I would closely watch and guide them.
At any time, it was completely clear that they were never allowed to take the knife without me actively giving it tothem. Not because I said so, but because it was my property, and not a shared item (like you would have with any random bread knife).
With this approach, you do know at every stage whether your child is ready or not, and it is very safe.