Look fellas, brings back a lot of memories....safety was the last thing on our minds..
We built our own and from others failures (lots of skin lost) we improved our designs.
Great stuff this thread....nowdays there are a lot more options.
As a now dad (I lived through this era) I try and make it (the billy cart) as safe as possible.
The driver on the other hand got no hope (kids will be kids)
Now onto your stuff....Steering is fine (rope is what we used).
Attaching the axle usually involved a heap of nails bent over/around the shaft.
Yours is lovley. Just add grease (kids love it).
To stop the head of any of the bolts pulling through use big washers top and bottom.
To stop the steering fulcrum bolt bolt leaving the scene .. big washers and we used to bash the thread of the bolt so the nut couldn't come off. Nowdays use a nylon lock nut.
Live while you're alive and sleep when you're dead