For what it's worth, generations of kids have grown up and learned to use knives with scout knives and SAK's, so I wouldn't be too focused on making sure that you get something with a lock. More whittling is probably done with a slipjoint vs any other knife. A SAK will hold up just fine to whittling sharp sticks. It will need to be sharpened at one point, but knowing how to sharpen a knife is part of owning a knife. Teaching the child the rules and how to safely use a knife is way more important than getting them any specific knife or making sure the knife has a good lock. There is also the logic that getting them a locking blade will allow them to learn to use a knife in ways that they shouldn't, i.e. abad habits and doing things and using the knife in ways that cause them to rely on the lock to hold the knife open, rather than focusing at using it correctly. There is really nothing that an 8 year old should be doing with a knife, that they can't safely do with a slipjoint. Just my opinion, take if for what it's worth.
And honestly all the suggestions that I have seen on here are good ones for a first knife, so I am not poopoo'ing anyone else's suggestions.
For a good set of rules, ones that have been thoroughly field tested, I would look to the Boy Scouts. Couple of my kids were scouts and I have watched a lot of inexperienced kids using knives. These rules work.
See this link:
http://usscouts.org/advance/cubscout/whittlingchip.asp
Once a Cub Scout (so starting at 5 or 6 years old) knows these rules and completes these requirements, they are allowed to carry a knife during campouts and use a knife as they please, as long as they are following the rules and being safe. They are guidelines that are just as valid outside of scouting as in.
Regardless of what kind of knife a kid uses, if they use it a lot, they are bound to cut themselves. It's unlikely to be anything that is life threatening or causes permanent damage. When it happens, show them how to correctly bandage it and talk about what they did wrong. Natural consequences are a powerful learning tool, so try not to be too worried about them or make too big of a deal.
Good luck!