You can build your kids a go-kart using readily available parts and a lawn mower engine. It doesn't have to cost an arm and a leg to build a go-kart. Many parts can be scavenged from the junkyard. Use larger dimensions for your kart if you want teens or adults to be able to ride it.
Build the frame. Cut the 3/4-inch water pipe into two 6-foot lengths and two 3-foot lengths. Join the corners with pipe elbow joints and weld together.
Attach the aluminum sheet to the bottom side of your frame.
Cut one 3-foot length of water pipe and two 1 1/2-foot pieces. Join these pieces with an elbow joint and weld together to make a second frame. Weld this frame to the first frame 2 1/2 feet from one end of the first frame.
Drill a 5/8-inch hole in the front pipe and attach the 1/2-inch mild steel bar to the pipe and to the second frame. Attach a steering wheel to the end.
Attach the front wheels via stubby axles pivoting on points allowing them to turn. Weld an angle to the steering shaft and hook this to the stubby axles via a universal type joint. Attach front wheels and brake assemblies either with a hand or foot brake.
Attach the 3/4-inch hardened steel bar below the first frame at the rear. You may use bushes or bearings, depending on how much you wish to spend, and attach the rear wheels.
Weld or attach a large bicycle cog to the center of the rear axle and ensure that there is sufficient clearance for the cog to spin freely to drive the axle.
Get two bucket seats from a junkyard and weld them in place behind the steering wheel in a comfortable place to sit.
Join five 3-foot pieces of pipe by elbows to create a third frame and weld three of them together. Attach this frame to the first frame by welding it directly behind the seats. Use the remaining two pieces of pipe to brace this frame to the first two frames.
Use a lawn mower engine, which has a small bicycle sprocket on the drive shaft and attach it to the first frame by welded and bolted supports so it is aligned to the cog on the rear drive axle. Attach the throttle cable to a pedal for an accelerator.
Attach a gasoline tank to the third frame above the height of the engine and use tubing for the gasoline to flow to the carburetor.
Attach a bicycle chain between the engine and the rear axle in a similar way as you would do with a bicycle. Fill the gasoline tank.
Prime the engine and have someone push and start you to take your first ride in your go-kart.
Always wear safety gear when driving a go-kart.