I made these for my son when he was a boy. They are made from 3/4 inch birch. They would make a good Christmas gift for a young male in your family.
Step 1: The tractor
The degree of detail is not for an exact replica, but suitable to stir the imagination of a young boy. Rather than give measurements I included a scale in inches in most of the photos. Begin by drawing the profile of the tractor body. The wheels were turned on my lathe. Axles are from 3/16 inch steel rod with end cap retainers from the hardware store. The exhaust stack is a piece of 1/4 inch welding rod for an oxy-acetylene torch. The steering wheel assembly is from a 3/8 inch carriage bolt with the square portion under the head ground away to make the column round. There will be more on the seat and the gear shift later.
Step 2: Rear of the tractor
In addition to the piece for the tractor frame, the other key piece (in addition to the wheels) is the axle member. I cut a notch in the underside of the tractor frame and in the upper side of the axle member to make a tight fitting lap joint. The red lines define the notch cut into the axle member. I drilled a 3/16 inch hole across the axle member that also goes through part of the tractor frame. When the 3/16 inch axle rod is inserted (yellow line), the tractor's main pieces are locked together.
Step 3: Front of the tractor
The front wheel assembly is in the fashion of a row crop tractor with each wheel canted inward slightly at the bottom. The wheel assembly is from a steel rod 5/16 inch in diameter with a 3/16 inch rod welded across its end to make an inverted "T". I bent the 3/16 inch rod ends downward a little to achieve the canted effect.