Toys.
When my kids were little, 30-40 years ago, I built a bunch of simple wooden toys with them. As they aged all their little kid toys were put away in storage in the attic. They grew up and moved away. When my wife and I relocated a few years ago and were packing up I noticed a bunch missing. Our kids had gone into the attic and gotten them for their kids.
When visiting for Christmas 2016 my son took his son out to my workshop and they built a replica of the aircraft carrier we had built when he was little, only bigger.
All extremely simple. They weren’t detailed, they were just enough to identify the object so their imaginations could do the rest.
This is similar to the carrier my son and grandson made 2x4 for the hull, can’t remember... maybe 3/8” ply for the flight deck, and a couple of blocks for the towers, about 4 feet long to accommodate my grandson’s toy planes. The one we made 35 years ago was about 2 feet long for smaller planes... like those little green army men size toys.
This is similar to the bulldozer. Clamped the belt sander on the bench and he rounded the edges of the 4x4 engine; although, a small plane would be good if you wanted him to gain experience with hand tools. The arms for the blade were 1x material, not 2x. 2x probably better, I remember one arm splitting and I had to glue it back... maybe should have used a straighter grained hardwood for that part.
No small dowels for exhaust pipe or controls, too easy to break.
No treads glued to the tracks, but a nice touch. I had drilled a couple of holes with a Forstner bit on each track to inset some wheels that extended about 1/2” below the tracks. I drilled a hole in the seat for a playschool person.
We didn’t make these but it looks simple. I would make the smokestack out of a thicker dowel to take abuse. I would maybe make the tops removable and drill out the engine compartment so he could put a little people in there.
Make the engine this time, add a car each time he comes.