Really little. It started before they were two when I used to run Train Simulator on my laptop. Alex used to love to blow the whistle. When the 2nd came along (2.5 years later) he too liked the Train Simulator. When the trains were finally built in Pennsy, I immediately let them couple and uncouple cars, follow the trains around, and touch anything as long as "Grandpop" was there to guide them. By the time they were five, they could run the controls. I had some loose trucks which they liked to shoot back and forth between them. We also did that with some sturdy cars. It was easy to do then since the layout had no scenery to get in the way.
We would also build lots of other stuff together including Legos and K'Nex. I also play video games with them and show interest in what they're interested in.
This is the same way I worked with my son and daughter when they were that age. I was building Pocher 1/8th scale model cars on commission. I was building three-at-a-time on the dining room table so there were cars and thousands of parts just sitting there. So whenever I was building, I would sit my son down next to me—he was just over two at the time—and give him some pieces to fit together.
I also had a part that both kids did. The vinyl tires were very thick and stiff and if you tried to put them on the wire wheel rims cold, the rim would disintegrate. So we had to heat them up in hot water. So one kid would put them in the water and one would take them out and then I'd build the wheels. Whenever I started a new batch they'd be asking if it was time to make the wheels yet.
In other words, and I apologize for this very long answer to a very short question, I've always believed that kids can touch things with supervision.
Now the kids major work is to put down ground cover and plastering.
