My 3 year old used a blade to open the processor box factory seal, and put the CPU in. This included opening the lever, removing the CPU cover from the motherboard and putting the CPU in place with my guiding hand to line up the notches.
My 5 year old used the blade to open the case box, and also installed the RAM. This included opening the side locking tabs, putting the RAM in (the wrong way haha), checking the orientation of the notch and turning the module around so it lined up properly. Then we pushed down hard to install it properly and the side locking tabs snapped into place.
And I have now finished setting it up, the PC is running like a charm.
Obviously they were sitting on my lap and we did it together (especially the blade bits, you should have seen the look on mummy's face! ), but they were able to do most of it themselves. It's just like a big puzzle. If they can do it, you can too!