That is a great moment for both of you! Thanks for sharing. What a nice locomotive to get to open!
It reminded me of a time I was with my grandfather and we built one of those pine airplane kits at his kitchen table. At one point, a small nail was needed to hold the propeller on the front. I had my pocket knife with me and I suggested using that to bang the nail home. My grandpa laughed and taught me a little lesson that has stayed with me since I was a little boy. "Use the right tool for the right job." We went to the garage, picked up the hammer and within a couple hits had the plane completed.
Great job including your little guy in the process. There is nothing better than doing something with your grandpa. He will remember it forever.