Best of luck to you! I had hoped my boys would get bit by the carving bug. In fact MY interest was sparked when they were in Boy Scouts and I thought I'd help em on the carving merit badge. Well, they both dropped out of scouts after a hot dirty summer camp experience. Seems they're both more interested in staying in the AC and playing video games or (more positively) - playing bass & drums. Guess I can understand where they're coming from but still a little disappointed.
Anyway - last summer my wife and I attended NEWR (Northeast Woodcarver's Roundup) in Honesdale PA and my youngest (Celine - 13) decided she'd like to come along. She LOVED it. She's our artsy crafty child (we have 4) and I was THRILLED. It's a great feeling to be carving with your child, and I hope someday to do the same with grandkids.
Lastly, I know everyone says to start them off with soap carving. From a learning & safety perspective I guess that's a good idea. But if it were me - I'd try Balsa wood maybe, or even a dense styrofoam. Why? Well for me - I HATE the feeling & smell of soap. It would totally turn me off if that's what I was given to start on. (Don't read into that by the way - YES I shower everyday but I use a gel - NOT soap. <grin>).
Just a thought. I hope your grandsons get bitten by the carving bug and you're able to enjoy many years of making chips with them.

ChuckT