My first blade when I was 8 was a smallish 2-bladed SAK style that required two hands to open the blades. 2 hand opening for a kid means less chance of an accident even though getting the finger nail in the slot was sometimes tricky. The main blade was about 2 1/2" long. It was similar but smaller than the one my Dad carried - but I loved it. Today, I would recommend a locking 2-3 implement SAK knife or a lockback single blade as a first knife.

When I was 9 I purchased a hobo style knife/sppon/fork/u-name-it with my allowance after going out on a camping trip. It eventually rusted out from being left in the rain. It was way too big for pocket carry. Later I inherited a 3 1/2" fixed blade /w sheath that I still have in my collection.

BTW, I second the advice from bacpacjac -- to a child it doesn't make sense until you explain the reasoning behind using a sharp knife versus a dull one. The more effort it takes to cut or whittle the better the chances you will slip and the knife will cut your hand or other body parts.