I started the young one on building some Snap-Site non-glue models when he was about five. I had them playing with the trains before that. My philosophy is: I don't forbid them from doing things (realistically… working with acetylene torches would be forbidden…) but, certain things must be done with Grandpop's help and guidance. When it's not forbidden, kids are less apt to sneak and do stuff without supervision. I let them roll cars back and forth and taught them how to use the manual uncoupling. I had some separate RR trucks that they'd roll back and forth to each other and got them used to re-railing trains. By age five or six they started being able to use the throttles with me hovering closely.
The face of my youngest grandson (now 14) is the most description picture of pure joy I can imagine.
When my son was two, I was building, in mass production fashion, 1/8 scale models of classic cars made by Pocher. I was building three-at-a-time on the dining room table and couldn't put the stuff away so I had to trust that my daughter (5) and son (2) wouldn't mess with it. These were the first three. Eventually, I made 65 Pocher kits on commission.
So I included both of them in the process. I would sit Adam next to me and give him some small nuts and bolts and other parts that he could assemble and disassemble. The cars had real spoke wheels which were very delicate. Onto them you had to 'force' thick vinyl tires. I found the only way to do it without wrecking the wheels was heat them in hot water and put them on. One kid dipped the tires in the water and one would take them out with tongs, and I put the on the wheels. They couldn't wait until it was "time to put the wheels on". I never had a single model project disturbed or damaged by the kids in all the years I was building stuff. Same goes for the grandkids.
Make them part of it. Teach them how to interact with tools and things. Don't forbid, but guide. Kids (most of them) don't want to wreck stuff unless they're frustrated.
My uncle with the HO trains in the 1950s, didn't have his own kids. He didn't have trouble with little kids, but couldn't handle teenagers. All he could say when I was with him with his trains was, "don't touch this…don't touch that." I was (and still am) a kid who looked with his hands as much as his eyes. It frustrated the heck out me that I couldn't touch his stuff. Meanwhile, by that time, I had my own permanent train layout up in the basement in which I did all the track laying and wiring, and I had been model building from when I was 8. It made no sense to me that I couldn't touch his trains. He didn't understand that Kids don't automatically mean breaking stuff.
Morale: Start them early and keep at it.
